I'd like to point out, as an aside, that storing the firearm in the trunk is unnecessary if the firearm is unloaded and in a "closed opaque case or secure wrapper". There is no requirement for a location, distance, or "steps" from the firearm. Not sure if you were implying that the gentleman just normally did that out of habit or because you thought it was required, so I wanted to clear that part up.
As for who takes possession, that's a good question. I would assume the driver could take possession of the pistol for the ride, but it would certainly be a grey area. This is an ambiguity best left to a lawyer's opinion.