Unfortunately, your friend is probably the 'back-up boyfriend'. She probably doesn't actually like him, but feels as though she always needs to be in a relationship, so when a relationship is over that she actually liked, she goes back with him just to have someone. I knew one girl like this, and her situation got pretty bad.. There's not a lot of hope for people like this; the best thing is to make the choice to respect yourself, and leave her. She has to learn on her own.