Is warning people about a man who is known to commit evil openly, and is known to people for that, regarded as backbiting that people will be questioned about on the Day of Resurrection?.
If the situation is as described and he mentions him so as to warn people against his evil so that those who do not know about him will not be deceived by him, it is permissible, but if it is only done for fun and to pass the time and so on then it is not permissible.
And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote.
Standing Committee for Academic Research and Issuing Fatwas