Dude, if she does break up with him for you she's a bad person. In fact she is already a bad person for going behind her BF like that playing around with you, so why the heck do you even want to be with a girl with such questionable morals ?
Just forget about her and her broken ethics, she's not worth it. You should be able to see that for yourself, even if she does start with you she will just start playing around with someone else behind your back too.
Just forget about her and her broken ethics, she's not worth it. You should be able to see that for yourself, even if she does start with you she will just start playing around with someone else behind your back too.