If she'll screw behind current BF back, then she'll cheat on you, too. If she wants to rationalize that it's ok to see other people behind her current boyfriends' back then she'll do the same to you when/if she isn't happy.

If your feelings are still strong, do it only if she can decide to leave the current relationship. It wouldn't be a good idea to endorse cheating behavior in her...for your own sake.