Well, in the conventional "macho" sense of it all a "man" would whip the guys ass. However, that same "man" would then get into all sorts of legal trouble and potentially ruin his future all for a girl he didn't know was the "one". How long have you been with this girl?
Honestly, Ryoku gave you the best advice in here. I'm not here to judge whether or not you girl is telling the truth.... I don't know her or the situation, so don't think I'm doing that. But this is something I've been through.
Here's a little background: I dated this chick for 5 months (we've been broken up for 2 now so this is fairly recent). I always got this weird feeling that she didn't trust me even though I did everything in my power to help her trust me. 3.5-4 months in the topic (trust) comes up in conversation. She tells me point blank she can't trust guys. Why? She got raped. Did I feel bad for her? Of course I did. However, it pissed me off to no end that she waited until I got real feelings for her before she told me. So I find out she can finally trust guys now..... after we broke up. It killed me because I put in all that hard work to show her not all guys are a$$holes only for some new guy to get the new-improved version of her. If it hadn't have been for her trust issues (the cause of MANY fights) this girl would have been close to perfect. Honestly, I'm glad she's better now, she's had a hard life and deserves better, but I'm mad as hell I had to be the transition guy. I never did one bad thing to the girl but I had to pay for what other guys before me did to her. If I had known all this going in, I may have just cut her loose and tried to help her outside of a relationship. I'm not saying I regret her, I'm just saying I invested a LOT and only got hurt in the end.
Straight-up this isn't something I like to talk about because the whole situation and how it played out really hurt me. Just be prepared. If you do decide to go for this girl it's gonna be one hell of a roller-coaster ride. So ask yourself "Am I really ready for this?" and then ask yourself if you really care enough about this girl to do through it all. Finally, if you do go through with it prepare for the possibility that you'll get hurt and come out the loser in this deal.