You can tell him, confront her. But more than likely, what will go down is this.... She denies all, he says bullshit and don't believe she'll do anything like that and you're just trying to mess up his relationship. They talk, she convinces him you're bullshiting. No one believes you and they ignore you. Then he'll apologise days, maybe weeks after they break up. I personally would stay out of it. Point him to this thread though and walk away saying nothing