if her bf cheated on her then there are two possibilities, either she actually likes you or she's just looking for a rebound.
the main Q is, do you actually like her??
sometimes if you think someone likes you and it turns out they're just using you as a rebound you can feel cheated on yourself.
if you don't like her just say it tho, and have your mates w you too, if you tell them not to laugh about it then she will know you mean business.
no matter what she thinks or how she behaves, don't feel forced to get into a relationship w her.
if u have a gf of your own its an easy decision, but the important thing is you have control here.
use it wisely (but enjoy it a little!)