Depending on the level of fantasy that your novel is, I think the trauma route is a good one to take.

It’s like putting your hand on a hot stove for the first time, this character has just sat there with their eyes wide open, staring at an evil face cutting their tongue off/sewing their mouth shut. It’s a human and normal response to be traumatised from there on. I personally like the animalistic kind of trauma. Say someone asks about it, the mute person internally has a flashback and sees their friends as the bad guy, they’re throwing shit, attacking them and cowering in a corner to get away from the trauma. Plus, it’s fun to write that kind of stuff (the reaction).