What is the ruling on remaining silent about condemning backbiting that is heard, even if one is not in the gathering where the backbiting occurred? And does an unbelieving woman differ from a Muslim woman in this regard? And how does one repent from that?

Denouncing evil is obligatory for whoever has confirmed its existence by hearing or sight, due to the Prophet's (peace be upon him) saying: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." Backbiting is forbidden, and the disbeliever is addressed by the subsidiary rulings of Sharia. So, if you hear someone backbiting a specific Muslim with evil, you must gently denounce him, unless you fear a greater harm. Repentance for not denouncing evil when it was obligatory is like repentance for all other sins.