Backbiting
Question : It has been written in your website that it is permissible for believers to commit backbiting in those cases in which the person sins publicly. Does this mean that only criticism in respect of him committing such sins openly can be done, or does backbiting him regarding all his sins and misdeeds become permissible?
Answer : When a person is committing sins openly and in public then his personal sanctity is no longer recognized and his backbiting is then allowed, even in respect to sins that he does not commit in public.
Backbiting a Believer
Question :

It is prohibited in Islam to backbite a believer; does this apply to a person who says that he is a Muslim when asked but he openly commits sins, such as consuming unlawful food and drink, being in forbidden relationships, etc.?

Answer :

If the believer commits sins openly then it is allowed to backbite and criticize him.

Prohibition of Backbiting
Question :

Backbiting is prohibited in Islam. So how can a man raise his voice against a cruel person or tell other people about the wicked nature of any person or system? If the wicked person was an oppressive ruler, is backbiting such a person permitted?

Answer :

Yes, it is permissible to backbite a person who is openly committing sins, and amongst the greatest of sins is oppression and aggression.

Backbiting a non-believer
Question :

Is it allowed to do gheebat (backbiting) of non-believer, i.e. someone who does not believe in imamate of 12 imams?

Answer :

It is not prohibited, but it is better to avoid this, since backbiting (gheeba) in most cases is without any benefit.