Warning people about cheaters is not backbiting
3-1-2018 | IslamWeb
Question:
Assalaamu alaykum. We have come here in India for medical treatment, and my father was explaining his problem and treatment until it reached the point of gossip. People usually use translators because they do not know Urdu. So he was explaining about them how they trick people, obtain money, etc. even though they are Muslim, but they have gone as far as to lie, be treacherous, rob people of their money, fool others, etc. So my question is: have I sinned because they are Muslims and we have talked badly about them? My parents told me that I should know about these people so that I would not get tricked by them. It is permissible for six conditions to gossip, but I do not know whether I have sinned or not. May Allah reward you.
Answer:
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad 
is His slave and Messenger.
In principle, it is forbidden to backbite a Muslim, but this may be permissible for a valid reason in the Shariah, such as warning against a deceiver.
Therefore, if those translators openly commit their evil actions or if it is necessary to warn against them, then there is no sin in backbiting about this, and you are not sinful for listening to your father while he is backbiting without forbidding him to do so.
Ibn Hajar Al-Haythami said in Az-Zawaajir ‘an Iqtiraaf Al-Kabaa’ir, “In principle, backbiting is prohibited; and it may be obligatory or allowed in certain cases for a valid purpose that cannot be achieved otherwise.”
An-Nawawi 
said:
“Know that backbiting is permissible for a valid Islamic purpose that cannot be achieved otherwise, which is one of the following six reasons:
1- Complaining against injustice: It is permissible for the oppressed to appeal to the ruler and the judge, or to any other person who has authority or ability to bring him back his right from the oppressor. So he says: so-and-so has wronged me with such-and-such.
2- Seeking the help to change evil and bring the sinner back to the straight path; so he says to those who are able to remove the evil: so-and-so does such-and-such evil, so repel him from this, and the like, and his intention should be that of removing evil. If this is not his intention, then it is prohibited.
3- Seeking a fatwa: He/she says to the Mufti: My father, or my brother, or my husband, or so-and-so wronged me by doing such-and-such, does he have the right to do that? How do I get my right from him and repel his injustice? And so forth, then this is permissible for a need. But it is better that he says: What would you say about a man, a person, or husband who does such-and-such? Here, the purpose is achieved without specifying or identifying the person who wronged you; however, identifying the person is permissible...
4- The fourth is warning the Muslims against evil and giving them advice, such as mentioning the flaws of weak narrators of Hadith, and the unreliable witnesses (in court); this is permissible according to the consensus of the scholars; rather, this is an obligation for a need.
This includes consultation regarding marrying one’s daughter to a man, undertaking partnership with him, keeping a trust with him, and so forth, or residing near him [being his neighbor]. The person who is asked for advice must not conceal his condition; rather, he mentions the disadvantages that the other person has with the intention of giving advice.
It also includes seeing a person who seeks knowledge going to a person who commits Bid‘ah (religious innovations) or a dissolute person from whom he takes knowledge and he fears that the person seeking knowledge would be harmed by that, so in such case, he must advise him by mentioning the truth about the person whom he goes to, provided that he intends giving advice by that. This is something people make mistakes in, as the speaker may be envious, and Satan lures him and makes him think that this is advice; so one should be heedful of this.
This also includes if a person has an authority [a position of leadership and responsibility] that he does not fulfill as required, either because he is not suitable for it, or because he is dissolute or heedless, and so on; it should be mentioned to the one who has a general authority to remove him and appoint someone else instead of him who is suitable for the position and who knows better than the former so that he deals with him according to his condition and is not lured by him, and he should urge him to be upright or replace him.
5- The fifth is if a person is openly committing immoral deeds, such as drinking wine, confiscating people’s property unlawfully, collecting illegitimate taxes, exercising unlawful authorities, etc., or following religious innovations. It is permissible to speak of what he is doing openly, but it is not permissible to speak of other flaws, except for another reason of those we mentioned.
6- Identification: If a person is known by the nickname: Al-A'mash (blear-eyed); the lame, the deaf, the blind, the cross-eyed, and the like, then they may be identified by that, but it is forbidden to say this as a way of degrading them, and if it is possible to identify them with other than this, then that is more appropriate.
These are six reasons mentioned by the scholars, and there is a scholarly consensus on most of them. The evidence for them is mentioned in authentic and well-known hadiths.”
Allah knows best.