Sounds like the classic 1/8 tank issue!
He ran the truck out of fuel, should not really happen going down hill, but that is what it sounds like.
You will need to bleed the system of air.
Start at Fuel filter if still in the stock location, or the inlet to the IP if the filters are at the pump.
Loosen the lid of the filter or the line to the IP and "bump the starter" you should get a good stream of fuel. It might take a couple of times to get a good stream, tighten those connections.
If a stock filter, now loosen the connection to the IP and bump again, should get fuel, tighten that connection.
Now crack three if the injector lines, at the injectors, while cranking the engine for SHORT periods (about 10 seconds) watch for fuel at the injector and tighten as it starts to leak fuel. about the time you tighten the second line it will most likely start to fire, run rough for a bit, about 30 seconds to a minute, then be fine.
With a Gasser you can run out of fuel and restart by just putting fuel in the tank and crank a bit, not so with a Diesel! It will almost always air lock on you and you will need to bleed the system of air.
I very seldom let mine get below about a 1/2 tank, just because I don't want to have to do this.