Is listening to people's complaints and household secrets—given that she is a trustworthy confidante—considered backbiting (gheebah) or slander (nameemah)? And should she prevent them from doing so? And how can she atone for it if it is a sin?

You must distance yourself from companions who violate the honor of Muslims; because that is one of the major sins forbidden by Islamic law. Listening to backbiting (gheebah) and slander (nameemah) is forbidden, and it is considered aiding and encouraging those who commit major sins, making the listener a partner in sin. You must repent from listening to such things, and advise these companions, reminding them of the ugliness of their actions. The wronged person is permitted to complain only to someone who can remove the harm from them, and not merely to relieve themselves.