[Odd Job]

A tough one to call.

If it was a more modern diesel I'd say straight away, drain off. Better to be safe than sorry.
But if he can brim it with diesel, and only uses it on slow runs, not fast motorway type of journeys then he might get away with it.

£20 doesn't buy much fuel these days, so I'd get my pen and paper out and calculate how much has actually gone in. Then make a decision on how the car gets used.
If he decides to risk it, make sure he changes the fuel filter after he's run the bad fuel through and is running back on derv.