He wants to enter Islam but he cannot give up coffee when fasting!

Firstly: 

The greatest of blessings that Allah bestows upon His slave,
which is also a sign of his being blessed and his success, is that his Lord
guides him to His religion, and opens his heart to believe in Him and submit
to Him. Allah says (interpretation of the meaning):

“And whomsoever Allaah wills to guide, He opens his breast
to Islam; and whomsoever He wills to send astray, He makes his breast closed
and constricted, as if he is climbing up to the sky. Thus Allaah puts the
wrath on those who believe not”

[al-An‘aam 6:125]. 

Shaykh al-Sa‘di (may Allah have mercy on him) said: 

Here Allah says, describing to His slaves the sign of a
person’s being blessed and guided, and the sign of his being doomed and
misguided, that for the one whose heart is open to Islam and is filled with
the light of faith and certainty, that he feels at ease with it, so he loves
to do good and finds it easy to do it. This is a sign that Allah has guided
him and blessed him by enabling him to follow the straight path. 

The sign of the one whom Allah wills to send astray is that
his heart becomes constricted and closed, so it rejects faith, knowledge and
certainty, and his heart is filled with specious arguments and whims and
desires. So nothing reaches him, and his heart is not open to doing good
deeds. It is as if he is “climbing up to the sky”, i.e., as if he is asked
to climb up to the sky when he has no means of doing so. 

The reason for this is their lack of faith, because of which
Allah “puts the wrath on them”, because they have blocked the door of mercy
and kindness. That is the way that does not change, because the one who
gives (in charity) and fears Allah, and testifies to the best, Allah will
indeed make smooth for him the path of bliss, but for the one who is greedy
miser and thinks himself self sufficient, and gives the lie to the best,
Allah will make smooth for him the path to misery. 

End quote. 

Tafseer al-Sa‘di, 272. 

We ask Allah to guide this seeker and open his heart to
Islam, and to enable him to repent to Him and submit to His religion. 

Secondly: 

It should be noted that fasting the month of Ramadan is not
something to be taken lightly; rather it is one of the five pillars of Islam
and one of the great foundations on which this faith is built, as it says in
the well-known hadeeth from Ibn ‘Umar (may Allah be pleased with him) who
said: The Messenger of Allah (blessings and peace of Allah be upon him)
said: “Islam is built on five (pillars): testimony that there is no god
except Allaah and that Muhammad is His slave and Messenger, establishing
regular prayer, paying zakaah, pilgrimage to the House and fasting Ramadaan.” 

Narrated by al-Bukhaari (8) and Muslim (16). 

It should also be noted that uttering the twin declaration of
faith (al-shahaadatayn) and entering Islam means that the person submits
with his heart and physical faculties to the Lord of the Worlds, and that he
believes in Him and in what came from Him; he submits to his Lord and
accepts His rulings, commands and prohibitions, and he believes what He
tells us of stories. Allah says (interpretation of the meaning):

“We sent no Messenger, but to be obeyed by Allaah’s Leave.
If they (hypocrites), when they had been unjust to themselves, had come to
you (Muhammad صلى الله عليه وسلم) and
begged Allaah’s forgiveness, and the Messenger had begged forgiveness for
them, indeed, they would have found Allaah All-Forgiving (One Who forgives
and accepts repentance), Most Merciful.

65. But no, by your Lord, they can have no Faith, until
they make you (O Muhammad صلى الله عليه وسلم)
judge in all disputes between them, and find in themselves no resistance
against your decisions, and accept (them) with full submission”

[al-Nisa’ 4:64-65]. 

Ibn Katheer (may Allah have mercy on him) said: 

In the words “But no, by your Lord, they can have no
Faith, until they make you (O Muhammad صلى الله
عليه وسلم) judge in all disputes between them”, Allah is swearing
by His holy Self that no one believes until he makes the Messenger
(blessings and peace of Allah be upon him) judge in all his affairs. So
whatever he rules is the truth to which he must submit both inwardly and
outwardly. Hence He says “and find in themselves no resistance against
your decisions, and accept (them) with full submission” i.e., if they
appoint you as judge, they should obey you in their hearts and not find any
resistance in themselves against your decision and submit to it both
outwardly and inwardly. Thus they should submit completely, without any
resentment or dispute.

End quote. 

Tafseer Ibn Katheer (2/349) 

There are very many verses in the Holy Qur’aan which confirm
this principle and explain it. 

Thirdly: 

Once we have understood this important principle of faith,
which is submission to and acceptance of everything that comes from Allah,
may He be exalted, and submitting to Him both inwardly and outwardly, then
we will also understand that fasting Ramadan is an important pillar of
faith, and no person’s faith is valid unless he accepts it. So what we have
to do, when calling this seeker to the religion of Allah, is explain to him
that there is no problem that would prevent him from entering Islam; Allah
does not put any hardship upon His slaves in their religion; rather He makes
it easy for them and relieves them of what that may cause them of hardship
and difficulty . 

Is this man drinks coffee because of what he suffers of
headaches and migraines, then he can consume the sufficient amount of it
during the night, then when he begins to fast during the day, he should stop
doing that until the sun sets. He can also seek help by using the medication
that will help him with his headache or reduce its effects. 

But it is well known that if a sick person is affected by his
sickness during the day, and his sickness will be affected by fasting, then
it is well known that such individuals are exempted in the religion of
Allah. So if he gets a headache and needs to take medicine, and cannot fast
that day, he can break the fast for that day, then when the month has ended
he should make up the days that he did not fast because of the excuse of
being sick. As Allah says (interpretation of the meaning):

“O you who believe! Observing As-Sawm (the fasting) is
prescribed for you as it was prescribed for those before you, that you may
become Al-Muttaqoon (the pious).

184. [Observing Sawm (fasts)] for a fixed number of days,
but if any of you is ill or on a journey, the same number (should be made
up) from other days. And as for those who can fast with difficulty, (e.g. an
old man), they have (a choice either to fast or) to feed a Miskeen (poor
person) (for every day). But whoever does good of his own accord, it is
better for him. And that you fast is better for you if only you know”

[al-Baqarah 2:183-184]. 

But if a person wants to drink coffee because he loves it,
and he does not want to forsake his desire for it during the day for one
month of every year, then this person is not ready to believe and submit or
to adhere truly to any religion, whether it is Islam or any other, because
religion — any religion — requires its followers to be able to obey and
submit, and to give up the whims and desires of the self. This is the test
and it is the obstacle that many people fail to overcome, the obstacle of
whims and desires and going against what the nafs (self) desires. This is
what Allah mentioned in His Book to His Prophet, as He said (interpretation
of the meaning):

“Have you (O Muhammad صلى
الله عليه وسلم) seen him who has taken as his ilâh (god) his own vain
desire? Would you then be a Wakîl (a disposer of his affairs or a watcher)
over him?”

[al-Furqaan 25:43]. 

Fourthly:  

Allah, may He be glorified and exalted, is more compassionate
towards His slaves than a mother to her child. When a person turns to his
Lord, Allah turns to him and more so, as it says in the hadeeth narrated
from Abu Hurayrah (may Allah be pleased with him) who said: The Messenger of
Allah (blessings and peace of Allah be upon him) said: “Allah says: ‘I am as
My slave thinks I am, and I am with him when he remembers Me. If he
remembers me to himself, I remember him to Myself; if he remembers Me in a
gathering, I remember him in a gathering better than it; if he draws near to
Me a handspan, I draw near to him an arm’s length; if he draws near to me an
arm’s length, I draw near to him a fathom’s length; if he comes to Me
walking, I go to him at speed.” Narrated by al-Bukhaari (7405) and Muslim
(2675). 

So be confident, O slave of Allah, that when you turn to
Allah sincerely, Allah will turn to you and will help you, make things easy
for you, open your heart, suffice you against what distresses you and
support you in ways you never imagined. Perhaps Allah will heal you of your
sickness, or perhaps He will suffice you and relieve you of that addiction
to what you drink [coffee]. So turn to Allah and think positively of your
Lord, and tell Him of your need. But the condition is that you should be
serious about submitting to the command of Allah and accepting what has come
from Him. Allah, may He be exalted, says (interpretation of the meaning):

“As for him who gives (in charity) and keeps his duty to
Allaah and fears Him,

6. And believes in Al‑Husnaa [The Best, (i.e. either Lâ
ilâha illAllaah: none has the right to be worshipped but Allaah) or a reward
from Allaah (i.e. Allaah will compensate him for what he will spend in
Allaah’s way or bless him with Paradise)],

7. We will make smooth for him the path of ease
(goodness).”

[al-Layl 92:5-7]. 

Fifthly: 

It should be noted that if a person is sincere in his belief
in his Lord, then is unable to do some of the duties, or he is overwhelmed
by his nafs and whims and desires, and falls into sin, that is not the end
of the road. Rather the gate of repentance and turning to his Lord is open.
Moreover, he — despite his sin — may be forgiven by Allah and has to hope
that his sin and error will be pardoned. But the problem for which there is
no solution is if a person remains in a state of shirk (associating others
with Allah) and turning away from the religion of his Lord. Allah says
(interpretation of the meaning):

“Verily, Allaah forgives not that partners should be set
up with Him (in worship), but He forgives except that (anything else) to
whom He wills; and whoever sets up partners with Allaah in worship, he has
indeed invented a tremendous sin”

[al-Nisa’ 4:48].

Sixthly: 

It is very important to note that it is not acceptable for
there to be a relationship or friendship between a man and a woman in Islam;
rather this is one of the actions of jaahiliyyah (ignorance) and the Muslim
should be above doing that. Islam forbids free mixing of men and women, men
entering upon women or being alone with them. 

This has been explained previously in numerous answers under
the heading “Relationships between the sexes” on this website; please refer
to that. 

And Allah knows best.