I don't know, Ben. If she really loves you would it be SO important for her making him regret? So, she needs the satisfaction that he knows what he lost to "finalize him", to have closure? Sounds weird, even for a 21 years old. Sounds actually like an excuse. Either she has some major ego-issues, or she is simply not done with him yet. To be honest, if I would be you I would simply give her an ultimatum... either him or me! I would not talk to her anymore, not hang out with her, consequently NOTHING of it. Before I would tell her exactly why you're doing it.. but then , as i said, zero contact. I also would give myself a time limit, for how long I can stand this status quo. And if she is not shocked and still worries more about his regret and her satisfaction than about hurting and losing you....... then she's not worth all the excitement. Then it comes down to it that, like her ex, she doesn't know what she is going to lose - you. But before she even can know what she really wants she must 'get the chance' to miss you. Do you understand what I mean?