I just think, what if you were to start falling for her? And what if she actually left her boyfriend for you? She'd probably cheat on you, too, if she's already cheating on him. People don't really change like that IMO--once a cheater, always a cheater.
Even if there's no chance of a relationship developing between you two, I'd get out anyway. What if the guy finds out and he comes after you or something?