1988 ford f350 won't start after killing it?

Chances are he's done major damage to the engine electrics. Wait a few days for it to dry out and see if it'll start, otherwise it'll need to go to a garage, possibly the ECU has been damaged. NEVER power wash an engine unless it's absolutely caked in oil and you're trying to find a leak.

I assume the weather has been the same there as elsewhere in the UK since you washed it - wet! Ideally, you should tow the car somewhere you can put it in a heated garage & give it 24 hours to dry out. Failing that, you'll have to start unplugging, cleaning, spraying & reconnecting any connectors under the bonnet.

If that doesn't work, it's a garage job. If he's managed to get water into the inlet side of your diesel engine, you could be looking at a new engine.