For me it's pretty blatant that she loves the attention of having two guys more or less fighting over her. If I were you i'd punish her for being an immature little girl.
Quite simply i'd back off, show her no attention, and when all she's getting is a needy girly guy giving her attention, she'll soon realise what she's missing, and most importantly realise that you aren't her's to fuck around with - remember, you are the prize.
I also recently had a situation like this. I didn't text the girl for 5 days and she soon came running back.. however I decided that I really didn't want to persue with her anymore, and she finally text me saying "Blergh. Cba." My point being that she will run back to you, i'm 100% sure of that.
Get chatting to other girls and make sure she knows about it, DHV my friend!
Oh and personally, this guy sounds like a prick.. don't feel like you're losing the battle between you if you "appear" to give up, cause he definitely won't be getting her. At the end of the day you either smash her in the long run and get your own back, or things fizzle out, you don't speak to her, and you do the guy over for being such a twat. (the last bit of obviously optional.. i would though!)
