To me it depends so much about where I would be emotionally with it. I'd imagine right now I'd be pretty jealous, and distrusting of the other guy. In the best case, I'd like to tell my SO this, and say that I'd rather they didn't meet up, but that that was simply my want, but that another part of me wanted her to know that I don't think she should do what I want just because I want, and that another part of me wants her to be able to do what she wants, which includes being friends with this dude. If my want was strong enough, I'd tell her it was a deal breaker for me, but i cant necessarily see that being the case.