With out knowing the specifics how can one make this statement?
He can't, but that doesn't stop people on this forum.
The REAL answer is: It depends.
It depends on HOW MUCH weight, HOW FAR behind the axle...
It's perfectly okay to hang 1000lbs of weight on the receiver hitch, which is roughly 4' behind the rear axle of the truck.
At worst, the CG of this 9'4" camper is 12" behind the rear axle of the truck. It would have to weigh 4000lbs to have the same effect on handling as 1000lbs on the receiver.
2002 Chevy 3500 DRW 8.1L/Allison
2000 Palomino B1500
...and the reason why I need a DRW to haul a Palomino:
2004 United 7x14 tandem axle enclosed toy trailer
2011 PJ 8x20 7-ton deckover equipment trailer
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!