I'll deal with the tailgate issue in a separate response...
IMHO, using the tailgate to support the camper is a BAD idea. A tailgate is designed to contain, not support.
Sure, it'll support a couple of heavy guys sitting on it at a party, but consider that with the camper, those "two heavy guys" are bouncing up and down on the tailgate for mile after mile after mile... How long will it last?
I've had tailgate cables snap on me in the past. It's no fun to be unceremoniously dumped on the ground, and you end up with a dented tailgate out of the deal...
It's not something I would do, if I could avoid it. I'd have to think there are some used campers out there DESIGNED for short bed trucks that your friend could have bought instead of the long bed one he bought.
2004.5 Dodge Ram 2500 CTD, Long bed quad cab, 315x70x17 Load Range E Firestone Destination AT's
1988 Lance 980 11'3" camper, fully self contained.. Hapijac Tie downs. Need to get the truck wired next!