Let's say a friend drives over and we want to go to the range, but I'm the only one with any firearms and we decide to take his car. Is it legal to have him drive with my firearms in the trunk?

if you are with him.. sure..

 

providing there are no odd undisclosed circumstances (ex: he is a prohibited person.. and even in that situation if they are being transported accordingly I am not even sure it is an issue.. not a door I would open.. but you get the idea..)

It's absurd that we even need to ask this question.