^(just me- but) I'd go large.
I've slept in the back of my wagon, and even in the back of my 944. If you inflate an air mattress in a wagon, it'll eat up a lot of the headroom, making it hard sometimes to sit up. When you hear something rummaging through the campsite at 3:00 am, you are going to want to be able to sit up.
The large tent lets you stand up which really helps if you need to stretch, or change clothes. Also good if it's raining and you are stuck "inside".
What I've noticed in my wagon, and in the 944, and in some pickups, is that it is hard to ventilate, without opening windows, which lets water/bugs in, and can catch too much warmth/condensation on the inside if the windows are closed.
Since this is essentially car camping- weight doesn't matter.
A while back I got this tent at a store. It was mismarked for about $60, so I bought it. It doesn't connect to a car per-se, but is so heavy, it will never get far from a car. Three rooms. Room to stand in main area. Friggin' huge. I use every square inch. Doesn't protect from snoring though.
https://www.baileysonline.com/grand-18-family-tent-3-room-10-ft-x-18-ft-x-72-spt-2260.html?gclid=EAIaIQobChMIu8OX6cfu6gIVgbbICh15LA VfEAQYCSABEgI4S_D_BwE
The large one off the back of the car would let you sleep in the car if it's raining, but be able to stand off the back and hang out/ dress. Gives a lot more privacy.
Make your friend bring his own tent!