In the U.S., it usually will depend on someones cooperation and the attitude of the cops. This is to say that if your friend was in the U.S., it's a fifty-fifty shot he'd be in any harm. If the police were to raid the place and the other guy copped to the plants, then your friend would most likely be let off the hook. On the other hand, if it was Officer Hard Ass, he'd take your friend down too...the old adage that you might be the rap but you won't beat the ride. If your friend is THAT worried, he probably needs to find another roommate.