He is mentally ill and does not pray or fast; what is the ruling?

Firstly: 

We ask Allah to heal this sick person from this sickness that
he is suffering, and we ask Him, may He be glorified, to guide us and him to
the truth and the right path, for He is able to do that. 

Secondly: 

The conditions of accountability with regard to commands and
prohibitions are being an adult and being of sound mind. When a person
becomes an adult of sound mind, it becomes obligatory for him to do what is
enjoined of praying, fasting, giving zakaah, performing Hajj and other
obligatory duties, and he is also obliged to avoid things that are
prohibited. 

It says in Fataawa al-Lajnah ad-Daa’imah (vol. 2,
6/370): Prayer is obligatory upon the individual and is not waived under any
circumstances so long as he is of sound mind, because the conditions of
accountability are being an adult and being of sound mind. The command to
pray is not waived because of loss of a limb or physical faculty, or because
of sickness, or similar problems, because of the general meaning of the
Qur’an and Sunnah, and scholarly consensus to that effect. But if a person
suffers some physical harm and is not able to do all the obligatory actions
and essential parts of the prayer, then he may pray according to what he is
able to do. End quote. 

Based on that, if this mental illness does not affect the
person’s reasoning or consciousness, as appears to be the case in the
situation mentioned, then it is obligatory for him to pray and fast, because
he is still accountable. 

But if he loses his mind sometimes, and is of sound mind
sometimes, then he is excused at the times when he loses his mind, but when
he comes back to his senses the excuse is no longer applicable, and he has
to offer the prayer that is currently due and make up the prayers that he
missed during the time when he lost his mind. 

What you must do is advise this man and remind him of the
great importance of prayer and fasting, and tell him that the one who does
not do these duties without any excuse is in grave danger; in fact keeping
away from remembrance of Allah is one of the greatest causes of mental
illness, as Allah, may He be exalted, says (interpretation of the meaning):

“But whosoever turns away from My Reminder (i.e. neither
believes in this Quran nor acts on its orders, etc.) verily, for him is a
life of hardship”

[Ta-Ha 20:124].

Allah, may He be glorified, also tells us of the means of
attaining peace of mind, as He says (interpretation of the meaning):

“Verily, in the remembrance of Allah do hearts find rest”

[ar-Ra ‘d 13:28]. 

It should be noted that the individual is responsible for all
of his deeds before his Lord, may He be glorified and exalted, so long as he
is aware of what he is saying or doing. His illness is not an excuse for him
to commit whatever sins he wants, such as neglecting the prayer, backbiting
and spreading malicious gossip. … The individual has no argument against his
Lord, may He be glorified and exalted; rather Allah has the perfect proof
and argument against all His creation. 

We ask Allah to set the affairs of the Muslims straight and
to bring them back to Him. 

And Allah knows best.