Like grubby alluded to: check the cap. It could be plugged or the wrong one altogether. I had a customer call me on a big Liftking once. They stated it would run great but after a while it would sputter and stall. They had the injectors and pump rebuilt with no change in the symptoms. I walked up to the truck, took a peek around, opened the fuel cap and "whoosh!", a huge rush of air as the vacuum in the tank was relieved. Someone had replaced the cap with a non-ventilated cap. Diesels need a vented cap because the lift pump won't be able to build enough pressure to overcome vacuum built up in the tank (due to the difference in the volume of fuel moving between the feed and the return). If you let it sit the vacuum might be finding a place to bleed off allowing restart after some time.

Also make sure your returns aren't blocked like he said too.

I've also found debris in primer pumps and lines but usually they're just dead with no chance of restarting... usually lol.

Tell your customer that when it happens again, remove the cap and listen for air rushing in and restart it with the cap off. Then order him a a new cap if that seems to correct the issue.