Wrongfully judging a person to be an apostate because he went out with a girl who was unveiled

Firstly: 

It is not permissible for a Muslim to use the word kaafir
lightly, because judging a Muslim who believes in Tawheed to be a kaafir is
a major sin. Muslim (60) narrated from ‘Abd-Allaah ibn ‘Umar that the
Messenger of Allaah (peace and blessings of Allaah be upon him)
said: “Any man who says to his
brother, ‘O kaafir,’ if this description fits him, then he is a kaafir,
otherwise it will come back on him (the one who said it).” 

Al-Bukhaari (6054) narrated from Abu Dharr (may Allaah be
pleased with him) that he heard the Prophet (peace and blessings of
Allaah be upon him) say: “No man accuses another of being immoral or of
being a kaafir but it will come back on him if his companion is not like
that.” 

Secondly: 

The one who wants to denounce an evil action or to exhort a
sinner should do that in a kind and gentle manner, because this is more
likely to make his words acceptable and effective. It was narrated from
‘Aa’ishah, the wife of the Prophet (peace and blessings of Allaah be
upon him) that the Messenger of Allaah (peace and blessings of Allaah
be upon him) said: “Allaah is Kind and loves kindness, and He gives (reward)
for kindness that He does not give for harshness and He does not give for
anything else. 

Al-Nawawi said: 

This hadeeth points to the virtue of kindness and encourages
us to adopt this attitude; and it condemns harshness. Kindness is the cause
of all goodness. Al-Qaadi said: What this means is that goals may be
achieved by means of kindness that cannot be achieved by anything else. 

With regard to uttering words such as kaafir (disbeliever),
faasiq (immoral person), murtadd (apostate), etc, this may be a cause of the
person being put off and persisting in his sin, and not accepting the
truth. 

Al-Haafiz said in his commentary on the hadeeth of Abu Dharr
mentioned above: 

This shows that if a person says to another, “You are a
faasiq (immoral person)” or “You are a kaafir”, and that is not the case,
then he is the one who deserves to be described in that manner. If the
person is as described, it does not necessarily mean that the one who called
him that is free of sin. If the intention is to advise him or someone else
by referring to his situation, it is permissible, but if the intention is to
expose and shame him, and and to hurt him, then it is not permissible,
because we are commanded to conceal people’s faults and teach then and
advise then in a manner that is better. As far as a person is able to
achieve that in a kind manner, it is not permissible for him to do it in a
harsh manner, because that may make him stubborn and cause him to persist in
that action, as is the nature of many people.  

Thirdly: 

Going with a girl who is unveiled and wears makeup is not
kufr, rather it is a sin because it is one of the means that lead to immoral
actions. But he should advise this person who has gone with a girl who is
unveiled and wears makeup, so that Allaah may guide him thereby.