What is the ruling on someone who backbit in front of me and I did not advise him, and am I sinful for forgetting exactly what he said, given my fear of the person he backbit in front of me? And how do we stop a teacher if he backbites, and what is better: for one of the students to go to him secretly after he has backbitten, or what?

One must repudiate evil when someone backbites, doing so in a good manner. If the questioner is unable to repudiate it, or if she repudiates it and he persists in backbiting, then it becomes obligatory to leave his company. Repentance from backbiting is obligatory, and it suffices for the questioner to seek general absolution from those she backbit, or to seek forgiveness for them and praise them with good. The general rule is that repudiating evil should be done immediately, but if there is a legitimate religious benefit in delaying the repudiation, then it is permissible to delay it and suffice with advising him secretly.