Am I helping her cheat on her boyfriend? What's the morally correct thing to do?

Honestly, I think the best thing to do for YOU, is to cut all ties. The longer it continues, the harder it will become. Her relationship was already in the dumps, BEFORE she sought you out. She needs what her current boyfriend cannot give to her. It's terrible, but some people have certain needs. That doesn't mean she needs to get it from you. Obviously, you have morals, if you didn't you wouldn't question it. Knowing it isn't going anywhere between you should help you in doing whats right. I know you feel bad for her, but in the end, you'll save yourself a lot of grief.