Heres the thing, you gotta assert dominance over the guy. Be sure to get them in the same room and then prove your are the alpha, if you are not physically fit I would suggest bringing a knife, or if it is legal where you live, a gun, however if you are more physically inclined, fighting him in hand to hand combat will probably more impressive to her. Once you beat him in a fight (murder optional) she will see that you are the better person for a mate and she will likely pine for you instead of him. You can pee on her to claim her as yours, but some girls are not into this and you should probably ask her first, lean from my mistakes. Tell me how it goes.