I've worked for both a Mercedes (commercial) and Iveco service agent.
I tended to work on the trucks rather than the vans. Nobody ever puts petrol in a truck, but when we had vans towed in it was simply a case of remove the tank, turn it upside down to drain it, refit and refill the tank, crank it over till it fires, go for a test drive, then tip the contaminated fuel in my Land Rover.
I'm not absolutely certain, but these were probably common rail engines and suffered no long term damage.
I run my Land Rover on LPG, so if I ever had a problem with the free diesel/petrol mix it would be easy to switch back to LPG until I got an opportunity to drain the tank.