Thats easy to explain, its your aircraft you both go flying together, he gets you to log P2 so effectively he has now become P1 for the purpose of gaining hours in the eyes of the CAA for future ratings.
If then queried by the authority at sometime in the future when he applies for a rating using illegally obtained hours, they check his log book its shows P1, they check your log book it shows P2, guess who is right in the eyes of the CAA ?
Not quite, he wanted to log his hours as P2.