If in a common area where he had equal access to them, just about anyone with such access could be charged.
Being charged is not the same as convicted. So, he might be charged but not convicted. It seems to me that he needs to find a better class of friends to avoid testing the theory on this one.
I have a rule ... (when I am off duty) if I get into a car and there are any police clubs, pellet guns, or marijuana pipes in the car, I immediately get out. It saves me a lot of trouble.