Friend v/s girlfriend

So im a really silent guy. Not because I'm an introvert but ig i usually lack witty replies to defend or even counter someone else's comments that they make about me or my stuffs. It's like a fight or flight thing for me. Either i keep my calm or i end up saying some really stupid, harming verse that i end up becoming the wrong guy.

I've been facing some really irritating people in my life these days. And the thing that irritates me the most is they're none other than my own people, close ones. One of my friend from college with whom my long term girlfriend talks to, he keeps on taking some friendly fights with her in chats and everything's fine until he starts joking or i'd say generating some real trust issues about me leaving her once i move to different city for higher studies. And although my girl is very resistant and strong against such mere talks but dude he simply crosses his line more often when he keeps calling her ugly and stuff. And it's so fkn irritating, i find myself helpless. He's a good friend but somehow i feel he's literally not respecting any boundaries.

I just don't know how to fix this mess. A mess which my friend cannot seem to understand cause some people are just so self centred. I see myself breaking the bond but then i doubt it. Cause i am just like that, fight or flight. Either i stay calm or i simply destroy everything. And idk why did i post this. Help ig?