The truth is he is not going to get rid of her because you tell him to. She is 17 and therefore a child. She’s acting like a child, and he is treating her like a child. Unfortunately, you cannot interfere because he is not going to listen to you. What’s worse is that he will resent your involvement. Stay out of it. Is he going to blow his apprenticeship? Probably. He has to decide when he is going to stop dealing with her and get a girl friend his own age.