^ In base trim, short cab, gas engine. Knock 1000# off that for a diesel engine, another 1000# or so for an extended cab/crew, a few hundred more for options and suddenly you're down to 4000lbs or so real world capacity. Manufacturers don't advertise using payload capacities for fully trimmed models. A Chev 3500 is rated for 14,900lbs GVWR in CCLB DRW configuration, with the 6.6Duramax (I just checked, they're up slightly from 09). I have axle weights from an 09 in that configuration (09 is rated for 11,600), which put it at 16,000lbs wet with a large sized Okanagan camper. The Lance he replaced it with weighs even more. So unless they've trimmed a bunch of weight out of the truck, a large-sized slide in will put a 3500 over weight.
Any of the large-sized campers will put a 3500 over GVWR wet/trip-ready. Also: axle weight is often overlooked. Just because it can carry 7000# doesn't mean it can carry that all on the back axle. The truck I referenced was scarily over the rear axle weight.
Either way, the point remains, a 2500 series truck will be too small if he's already looking at HD model trucks to begin with. Most of the guys on here running 1/2-tons should be up a class anyway, this site is chronically over-GVWR.