Is it obligatory to listen to friends' complaints that fall under the category of backbiting and offer them advice, or to forbid them from doing so?

It is not permissible for a Muslim to listen to backbiting. Instead, he must condemn it. If the backbiter does not stop, then one must leave the gathering, for Allah the Almighty says: "And when you see those who engage in [blasphemous] discourse about Our verses, then turn away from them until they engage in another discourse. And if Satan should cause you to forget, then do not remain after the recollection with the wrongdoing people."

Exceptions to this include seeking justice, advice, or reconciliation, provided one limits oneself to what achieves the benefit, and it is preferable to speak generally rather than in detail.