No reason that the truck can't tow a 5th wheel PROVIDED one is found that suits their needs for room and CCC but not many choices that have much of either of those things for that truck. Major limiting factor is the payload of the truck (as it relates to the PIN weight of the trailer). That is probably MORE of a problem than the short bed but both can be overcome by purchasing a trailer that fits the specifications of the truck. Casita makes a 5th wheel version but it's pretty tiny but maybe that's what they want. Over all there would be TONS more options available in a travel trailer and absolutely NO reason it wouldn't tow any of them well. A travel trailer equal to the size of the 5th wheel he will be able to tow would CERTAINLY handle as well as a 5th wheel because that's a tiny trailer.
As for mounting of the hitch, it APPEARS that someone is attempting to mount a hitch that doesn't overload the rear axles so he's probably NOT looking at a size he can handle.
Good luck / Skip