Me personally? I wouldn’t care in the slightest. If he’s not cheating and the girl isn’t a treat to the relationship, then I’d say that’s fine. If she did anything to threaten the relationship (say he could do better, make moves, etc) I’d have a big problem. BUT! My attachment style is secure, and I understand that my reactions to these things may be different than others. I’d say you might want to talk to him about boundaries if it’s upsetting you, especially since a snap back takes two seconds