Is it Allowed to Backbite Someone who is Defiantly Disobedient [a Faasiq]?
by The Albaani Blog
Questioner: Is it allowed to backbite someone who is defiantly disobedient [a faasiq] in order to warn against him and so as not to fall into trials [fitnah] like fornication and stealing?
Al-Albaani: Naturally, [that is] the least that can be said, otherwise it is obligatory.
Al-Hudaa wan-Noor, 12.