Assuming the individual stopped driving the car because it would no longer run.Or was getting quite cranky. The injection pump and perhaps even other things might be full of undesirable stuff.

I would fill the injection pump with a solvent like laquer thinner and see what I could pump out of the return line after twenty minutes soaking.Capture it to examine it. Lot of foreign stuff comes out soak it again. For both operations use the finger pump.

. Do not try to run the diesel engine on laquer thinner. Pump new diesel in first before cranking the engine over to displace all the thinner and other crap it may break down if in there.

You especially do not want to feed the crap through the injectors. Plus laquer thinner burns like gasoline. New secondary filter for sure as well. The laquer thinner is agressive enough to really dissolve problem buildups reasonably fast in comparison to many other solvents.

Some people have dumped almost anything into the fuel tanks.Properly done you can get away with it. Unfortunatly too many just believed the myth of just dump whatever they found in. Sure it will run but the question becomes how long.