This is just a suggestion; if you are PIC and you don't already have an understanding with your mate that he can do some of the flying on any flight, just say no. You're not just PIC because you are logging the flight as such in your logbook, you need to actively captain your aircraft, and there are real world captaincy skills which you will need to develop through knowledge and experience. However if you're mate is just a dick, then probably best to leave him on the ground like you say!