I feel you should suggest the guy gets his mix somewhere else as you would only give the correct mix and give your reasons. I would also question whether you should be diving with someone who is not responsible enough with their own safety - you might need him in an emergency and you need to feel confident in him.
I had a difficult discussion with my brother some years ago as he was just an occassional recreational diver whereas I had moved on to more regular technical diving and his standard of diving was very poor. I told him straight I would not dive with him again until he undertook further training and if he didn't want to do that he should pack in for his own safety........ He packed in diving.