Most Helpful Guy
My best suggestion would be to... wait, but don't wait. Keep being you and doing the stuff you love, go out and see and meet other girls, but don't necessarily actively seek out a relationship. Just remind yourself that there are other girls available, and see what happens. If she breaks up with him, great! You can put some more emphasis on getting her attention. But if you meet another girl that you have feelings for, pursue her instead.
In short: Don't wait for her and him to break up, go out and be your own person, but don't just snatch up a random relationship. Date again if you find another girl that you have feelings for, or if the two of them break up.