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."
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.
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.
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.