I thought about this seriously and I think I would be cool with it. Even if I was in love with the girl and she had a strong emotional connection with this other girl. I don't think I would mind. It'd be kind of just like her having a normal close female friend, except she was fucking her too. No problem. Although if the other girl was really unattractive and un-feminine I think it would maybe bother me. If she was just a normal girl who was decent looking I think I'd be totally cool with it.