The reasoning is because said girl had once tried to oust my girlfriend from their friend group, and has treated her badly in the past. My girlfriend doesn't trust her. But every time I see them out and about socially (I'm also a member of this friend group at this point), there seems to be no animosity there. They're always tagging eachother in stupid Facebook videos and engaging in perfectly polite conversation, online or otherwise. Two weeks ago I went to meet my girlfriend for happy hour and it was the two of them there at the bar. Nobody else. So because of this I really didn't think it was a big deal. My girlfriend has had no qualms about me talking to any other women in this group, just this one. I've even hung out with her friends independently of her multiple times. But this is a dealbreaker? I just don't get it.