A 13'-16' Scamp or a 16' Casita would work ok, but performance won't be stellar. I know of some folks on another forum (for molded-fiberglass 'egg' trailers) who've towed with that vehicle. A couple of them seemed pretty happy, and a couple of others subsequently switched to larger tow vehicles.
When you start looking at more squarish TTs, though, the wind resistance is greater. If your friend is looking at short trips not far from home, at 55 to perhaps 60 mph, he'd manage ok. If he wants to go to the mountains, he might be unhappy.