Backbiting for the right reason
Talking about someone behind their back is allowed if it's to protect or help someone.
A man warned Prophet Musa that the leaders wanted to kill him. He told him in private to help him escape.
Some companions told the Prophet Muhammad what an enemy said behind his back. Not to gossip, but to protect the Prophet.
The Prophet didn't get angry at them. Because their intention was to give a warning, not to insult or spread drama.
If you talk about someone to help others, to give real warning, with sincere purpose then that is okay.
That’s the kind of "backbiting" that counts as advice, not sin.
(from Syajaratul Ma‘arif, Syaikh al-‘Izz bin Abdus Salam)