I don't have much experience in this department bu I'm going to tell what's the right thing to do. I think you should leave this girl alone. This girl claims thats she's interested in you but she still has a boyfriend. I can tell from what you said about her that this girl has no attention of being in a serious relationship at all. If I were you, I would leave her alone. What kind of girl is in a relationship and is interested in another guy. This girl is playing you. There are plenty other girls in the world and one of them will be committed to being a honest and stable relationship with you. My advice to ask God for help in this situation and he will direct you into a good position.
TO GOD BE THE GLORY