Check this link.
http://www.fleet.ford.com/towing-guides/Click on the 2014 towing guide, and it will show the GCVWR for the F-250 diesel. Is it crewcab, supercab or the rare regular cab?
I would guess supercab, and look up the cargo rating on page 10 - it can carry about 2,100 pounds before going over the GVWR.
Then look up the GCVWR on page 16, it can have a combined weight of about 23,500 pounds. (same weight with either axle ratio) Take away the 8,000 pound curb weight (with a couple of passengers) and you have a towing ability of about 15,500 pounds.
So unless he has a small family and regular cab, the truck will be over it's GVWR by a few hundred pounds. Not to much to worry about. If he does not take along water, and keeps everything that is heavy behind the trailer axles, he should be fine.
Have fun camping!
Fred.