There is a valid reason for the frame warranty issue, the GN does increase stress on the fiver frame and no one can tell him IF his specific trailer frame design and welding is up to handling the increased stress.
Unless he frequently tows gooseneck equipment trailers (which is unlikely or he would already have a GN ball mount in his truck), there are lots of reasons not to use a GN ball mount. Safety chains are required with a ball mount but not with a fifth wheel hitch. Hookup is more difficult with the GN ball since it can't be seen from the driver's seat and alignment is more critical. Most GN hitches require climbing into the truck bed to set or remove the lock pin.
I haul a GN tool trailer several times each year, but use the fifth wheel hitch much more frequently. I'll keep the fifth wheel hitch and deal with removing it a few times rather than live with a GN hitch all the time.
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK