Tell him to load about 500 lbs of stuff in the truck with full tank of fuel and go to a scale and weigh the truck. The difference between his weight and the gvwr of the truck (posted on sticker in door jamb) is what he has left in payload for pin weight.
The pin weight of most fifth wheels will be around 20 percent of the loaded weight of the trailer. Ignore dry weights and what the manufacturer says the pin weighs. Loaded is what counts.
That sounds like too much trailer for that truck, but it will pull it.