Hey, we've all (or most of us) have been in your position, and we've all handled it differently.If you were to ask for my opinion, I would say..just be yourself, and if she really cares for you like you do her, then she'll fall for you if she breaks up with him.I kinda gave up on the whole love thing a while ago, but who knows, I might find it someday.But that's enough of me.This is about you.
Just verify if she really loves you like you do her, because if you end up compromising who you are for some girl who doesn't feel the same about you or is toying with your feelings, and you end up paying the price, it's your own mistake.But that's how people learn
I would give you some slightly different advice, but I can tell you don't want to just hit it and quit it.
Hope this helped bro
Btw, how long have you known her, because that kinda matters a bit.