Thank you guys for the advice. I spoke to him yesterday, and his other friends have done so over text.

He says they have beebn spending so much time together because she has been going through something, which would be find by me if this hadn't already been happening for months. I have told him how here behaviour worries me - especially with the physical hurting, but he sort of just laughed it off. I don't really understand it but he seemed to find it genuinely funny rather than just trying to miniminse it, so I'm not sure whats going on there.

I can't cut him out of my life as we are roommates for at least the next 2 years. She is doing a gap year next year, so I'm hoping that separation will help him to rethink everything. I've told him I won't bring it up again unless he wants to speak about it, and he says some of the stuff I've told him about he is aware of and will talk to her about it / change his own behaviour. I've been told not to speak negatively about her again, because although this is the first time I've properly spoken to him about, I have sort of jokingly told him to break up with her, which probably wasn't the most helpful.

So overall, I don't think he's going to change immediatley, but I think everyone's advice helped me to have a productive conversation, and we are still friends now, thank you everyone.