The third place where backbiting is permissible, which is the one who warns.

The Shaykh : "And the one who warns" is the third characteristic. I see so-and-so associating with so-and-so, so I go and say to him: "Hey, are you going to keep hanging out with so-and-so? May Allah do you good? This is his character, such and such." I am backbiting him; why? Because the purpose here is to warn the righteous young man, warning him not to mix with the corrupt young man. We forbid the young man of good character from mixing with the young man of bad character; why? Because the nature of people is that it spreads, like beautiful fruits: if they are placed next to rotten fruit, it spoils them, and the infection spreads to them. This is the law of Allah in His creation, .
And for that reason, the Messenger (peace be upon him) warned against the company of the bad, saying: meaning: he gives it to you for free. . So the companion of the righteous man is a gain in every case; his example is like that of the one who sits with the perfumer: either he gives it to him for free, or he buys it from him for money, or at the very least he is content with smelling the pleasant fragrance.
As for the example of the bad companion, he (peace be upon him) said in the completion of the hadith: . This and that are the example of the righteous companion and the example of the bad companion. So if we see a righteous person mixing with a wicked person, we warn him and say to him: "So-and-so, the one you are hanging out with, does such and such, says such and such, and believes such and such," and so on. We do not hesitate and say: "No, we do not want to backbite people!" This is not disliked backbiting; rather, this is obligatory backbiting, not just permissible. This is a warning: "Beware of going with so-and-so, because his character is such and such." This is obligatory.
So we now have with us three characteristics up to this point: the one who is wronged, the one who identifies, and the one who warns.