Simple solution to a simple problem. Just tell her you saw her bf making out with some other chick at the mall. She gets all depressed and starts crying. You show up with some flowers, chocolates, condoms whatever it is that girls like now-a-days and boom, now she thinks you're mr. perfect, you can probably catch her on a good enough rebound to get some the same night then all you have to worry about is her now ex-bf trying to find a way to fight you.

Ok, now for the serious response. If you really like this girl and see that there might be a future with her. Wait it out. Be patient. Girls don't like to jump into a relationship right after ending one. They think people will see them as sluts or something. Just wait it out and see how things unfold