Is it obligatory to denounce someone who backbites a Muslim if it is not known whether backbiting him is permissible (such as a wronged person) or not?

Backbiting is originally forbidden by the Quran, Sunnah, and the consensus of the Ummah. However, a forbidden act may become permissible under certain circumstances, and backbiting has situations that allow it.

If the intention behind speaking is, for example, about an injustice that befell someone, then this is permissible. However, if it involves slandering the one who wronged him and speaking about him in a hurtful way, then this is among the forbidden forms of backbiting.

It is necessary to advise those who backbite and clarify the prohibition of backbiting, and that it is only permissible in specific circumstances. If it is forbidden backbiting, you must denounce it if you are able, or leave that gathering if you can.

Imam Al-Nawawi says: It is obligatory for anyone who hears forbidden backbiting to forbid it if he does not fear apparent harm. Otherwise, it is obligatory to denounce it in the heart and leave the gathering if possible. If he is able to denounce it verbally or stop the backbiting with other words, he is obligated to do so.