I am adding this: If he is extremely lucky the gasoline loosened up a bunch of crap in his fuel system and it plugged up his fuel filters (if not the secondary filter will need to be changed anyway).
It all depends on how much diesel was still in the tank.
Where I used to work a customer came in with the same problem but he dumped a bunch of ATF Dextron into the tank; after which he drove 4 miles to us. He did not put 5 gallons into the thank. more like 1 1/2-2 before he realized he was putting gas in. He also had a 1/4 tank or so if diesel fuel.
In his case the result was that it burned off the parts of the injector spray nozzle pintal that was exposed to combustion.
Besides the injectors it can damage the IP Elements (Plunger & Barrels) and score up the supply pump piston.
He won't know until he runs it on diesel again; there may not be any obvious problem at all if he is lucky.
If after running it only on Diesel Fuel he has some problems the Fuel Supply pump pressure should be checked; if OK the Injectors pulled and tested.
If the injectors are OK and it runs erratically after that I would expect that the IP Elements (Plungers & Barrels) were damaged.

__________________
84 300D, 82 Volvo 244Gl Diesel