I don't recommend locking people in the trunk of your car. It's cramped back there! And what if you get pulled over? It is not easy to explain away a body in your trunk. And I seriously doubt at that point your friend will be willing to back up the whole, "I am NOT kidnapping this person to murder and dump in a ditch" theory.
Heck, I know any one of my friends would let me squirm at that point.