There are some NLP shit which are ideal for this situation. Talking to her try to find out what are the things that attract her towards this guy and then talk her into state where she`ll find YOU as the guy who conveys all that stuff she searches for.
For example, if she says he likes his good developed body, subconciuosly she seeks for the strong alpha male who can protect her. Then tell her a story (even if it didn`t happen)

when you saw a guy hitting his girlfriend and how you totaly freaked out and beat the shit out of that guy, tell her how only cowards beat women.
Or, if she says he has a good car, deep inside she is seeking for financial security which he can give her. Then you can tell her how you are doing well with your college which will provide you high income job, or how u invested some money into investment funds or whatever.
I hope you got the point. This will result with her seeing in you all the positive things which she is looking for in guy and since you are there and that obstacle guy is not, you will be in game my friend.
Anyway, that is NLP. My way would probably be to outalpha the guy, but thats just the way I am!
Good luck!