Feelings aren't straight-forward, unfortunately. She might be feeling for you because you're new and exciting, and someone she can trust (if you've been friends), but also still have feelings for her boyfriend/ex/whatever.
The earlier you set things straight, the better.. Ask her to choose between you and him, make a choice, and make sure she knows that unless you all three agree to have a three-way relationship, you are not sharing.
She might hurt you and say you had fun, but her heart is elsewhere. She might hurt him because obviously breaking up because she's seeing someone on the side isn't ideal. He might be mad at you. Also, though you may like her now, are you sure you want to get involved with a cheating girl? She could do it to you too in a years timeø
Tricky.