Is it obligatory to respond to a backbiter if it is known that the response will not benefit those who backbite and they will continue their backbiting? And what is the ruling if one is too shy to respond to them, despite being able to, and disapproves of that in their heart?

A Muslim must denounce evil to the best of their ability, even if it does not lead to its cessation. This is done by changing it with the hand; if unable, then with the tongue; if unable, then with the heart, which is the weakest of faith. It is obligatory to repel backbiting against a Muslim and to defend their honor, due to the great reward associated with that. Shyness must not prevent a person from denouncing evil or speaking the truth, as this is blameworthy shyness. The teacher can be diverted from backbiting by asking a question or gently interrupting their speech. If that is not possible, one should leave the gathering if able; otherwise, one should denounce it with their heart.