Praise be to Allah.
The one who is caught red-handed trying to defraud someone deserves punishment, and he must return what he took unlawfully. This is something that is to be decided by the judicial authorities, or by the departmental body that has the authority to question him.
It is not appropriate to expose him or tell everybody of his misdeeds, unless he did it blatantly and recklessly, or there is the fear that he could defraud or cheat more people, if people are not aware of him and if he appears to be persisting in committing his misdemeanours.
An-Nawawi (may Allah have mercy on him) said: With regard to the concealment that is required in this case, what is meant by that is concealment of those who appear to be good people and the like, who are not known to be evildoers or mischief makers.
As for one who is known for being like that, it is recommended that his situation should not be concealed; rather his case should be referred to the authorities, provided that there is no fear of any negative consequences, because concealing in this case will encourage him to do more harm and spread more mischief, and transgress the sacred limits, and it will encourage others to do likewise. End quote from Sharh Muslim (16/135).
Ibn Rajab (may Allah have mercy on him) said: If someone is well known for committing sins, and does so openly, and does not care what he commits of sins or what is said about him, then he is an evildoer who commits evil openly, and talking about him and his misdeeds is not regarded as backbiting, as al-Hasan al-Basri and others stated. With regard to such a person, there is nothing wrong with investigating and finding out about him, so that the hadd punishments may be carried out against him. That was stated clearly by one of our companions, and he quoted as evidence for that the words of the Prophet (blessings and peace of Allah be upon him): “O Unays, go to the wife of that man in the morning, and if she admits it, then stone her.” There can be no intercession for such a person if he is caught, even if the authorities are not informed; rather he should be left so that the hadd punishment may be carried out against him, in order to put an end to his evil and to deter others like him.
Maalik said: If someone is not known to cause harm and offence to people – rather he made a mistake – there is nothing wrong with interceding for him, provided that news of his case has not reached the authorities. But if he is known for evildoing and mischief, I would not approve of anyone interceding for him; rather he should be left so that the hadd punishment may be carried out against him. This was narrated by Ibn al-Mundhir and others. End quote from Jaami‘ al-‘Uloom wa’l-Hikam (1/341).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: What is meant by concealing is hiding faults, concealing is not something praiseworthy unless it serves an interest and will not lead to evil consequences. For example, if an offender commits an offence, we do not conceal him if he is known to be an evildoer and mischief maker. But in the case of a man who appears to be good, then he did something that is not permissible, concealment may be what is required in his case. So concealment depends on what interest is to be served; the person who is known for evildoing and mischief should not be concealed, whereas in the case of one who appears to be good but made a mistake, the Sunnah is to conceal his mistake. End quote from Sharh al-Arba‘een an-Nawawiyyah (1/172).
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If you only spoke about this man because he was shameless and known to cause harm to others, or for fear that he would not be punished if his situation was not made known to others, or for fear that people might fall prey to his evil and tricks if his situation was not made known, then there is no blame on you.
But if he was not the type to commit offenses openly and shamelessly, and your telling everyone about him will not help to ensure that he is punished or ward off his evil, then you made a mistake.
The fact that he is not Muslim does not make it permissible to harm him or hurt his feelings. Ibn Hibbaan included a chapter in his Saheeh entitled, “The Fire is the lot of the one who says things to offend the People of the Book,” in which he quoted the hadith of Abu Moosaa, from the Prophet (blessings and peace of Allah be upon him), who said: “Whoever says something offensive to a Jew or a Christian will enter the fire.” This hadith was classed as saheeh by Shu‘ayb al-Arna’oot in Tahqeeq Ibn Hibbaan, and by al-Albaani in as-Saheehah, no. 3093.
Ibn Hajar al-Haytami said in az-Zawaajir ‘an Iqtiraaf al-Kabaa’ir (2/27): al-Ghazaali was asked in his Fataawa about backbiting about a disbeliever.
He said: On the part of the Muslim it is forbidden for three reasons: hurting the feelings of a fellow human; belittling the creation of Allah, for Allah is the Creator of people’s deeds; and wasting time on something that is of no benefit.
He said: The ruling on the first is that it is haraam, on the second that it is makrooh, and on the third that it is other than what is more appropriate.
With regard to the dhimmi [a non-Muslim living under Muslim rule], he is like the Muslim in that no one should hurt him, because Islam has protected his honour, his life and his wealth. It says in al-Khaadim: The first reason is the correct reason [i.e., the first reason – which is that it is hurting the feelings of a fellow human – is the real reason why it is forbidden].
Ibn Hibbaan narrated in his Saheeh that the Prophet (blessings and peace of Allah be upon him) said: “Whoever says something offensive to a Jew or a Christian will enter the fire.”
What is meant in this hadith is saying something to annoy or offend him. This settles the matter, because it clearly indicates that it is haraam to do that.
Al-Ghazaali said: As for the harbi [a non-Muslim whose people are at war with the Muslims], it is not haraam with regard the first reason, but with regard to the second and third reasons it is makrooh.
As for the innovator, if his innovation constitutes disbelief, then he comes under the same ruling as the harbi; otherwise, he comes under the same ruling as a Muslim. As for speaking about him in the context of his innovation, that is not makrooh. End quote.
For more information, please see the answers to questions no. 13611 and 149306.
And Allah knows best.