Most Helpful Girl
Tell an adult. It is NOT a horrible thing to be doing. You are caring for your friend. Something bad could happen to her. This guy could also take advantage of her. It is against the law for him and her to be together.
First tell her that this is not good and that something bad could happen. Tell her it is against the law and that he could get in trouble too. If she ignores it and does not break up with him... tell the adult.
I could not think of a normal 19 year old wanting to date a 14 year old. It seems he is trying to take advantage of her.