Lets look at this from the girlfriend's perspective for a moment. IF she did find out about you two, it would hurt her and you and maybe him. Some men don't care and cheating is the norm for them.

If he can do this to someone he really, really, really likes, he can and in a way, is cheating on you as well.

Don't you think you deserve to have your own man? Afterall, HE chose her again after chosing you. That may be a test to see if you will tolerate this unacceptable behavior.

Gather your support around you.