First off, I was the girlfriend of a guy who cheated and its a horrible feeling. Think of how you would feel if it was done to you.
That said, I think you should back off. He’s cheating with you so he will cheat on you if you two ever become official. Tell him how you feel and then let him make up his mind. If he truly cares about you and wants to be with you, other than a secret hook up, then he will break it off with his girlfriend and be with you. If he doesn’t, then it was never anything more than fun.
And no I don’t think it is your place to say anything to the girl. She wont believe you and it wont help matters at all but instead make him probably mad and dislike you and she will be out to screw you over.