If you are flying with your PPL friend, [you] can not log the time because you will not have had the differences training that you need to be able to log time in this aircraft. He is P1, you are a passenger.
This maybe raises an interesting point. If he's not got a CPL, then is he allowed to do anything more than 50/50 cost sharing for the hire of the aircraft? I maybe had best no pursue that one....