Putting petrol in petrol into a diesel engine can be very serious. Petrol doesn't have the same lubricity properties as diesel fuel, which means more friction and heat. It could mean wrecked fuel pump, injectors, pipes, filters – or even a whole new engine in a worst-case scenario.

The only way to be sure that the engine is safe from damage would be to get the fuel tank drained and fuel lines cleaned. If your son doesn't want to do this, adding fuel lubricity additives to the tank regularly and keeping it above half full for 2000 miles might allow him to get away with it.

But your son will be taking a huge risk with his car. I would be inclined to get the fuel tank drained and cleaned by a professional.