You should expose him, let him be punished, and you should resolutely leave him. He has hurt you several times, so you should give him another chance to hurt you?
Leave him alone and expose him. Even if you don't tell her, her girlfriend will know sooner or later, because there is no airtight wall.
Really, don't make any more mistakes. Such a man can't be brought.
He's just using you. Don't be soft on him, because he doesn't deserve your kindness.