You do nothing unless she breaks up with the guy. And that depends on whether YOU like her as much-if you don't, then don't bother with her. But do NOT get in the middle of a couple who are still together-that never works out. Treat her like just a friend until she's single.
Well, you should tell her if she likes you more, she needs to leave him for you, cause she won't do that, then she is lying to you, just to make you think she likes you more. So I'd tell her to leave him for you. If she likes you so much more. Hope this helps.