On the VW diesels there is a flap (changeover valve/or N239 in VW speak) that closes on shut down. It completely strangles the air supply. It is supposed to lessen the typical diesel shudder as you switch off. So that should initially stop the run away. The only thing is that opens again after a second so it could all start up again if the engine is still turning! A close shave. Assuming it is still running on all 4 cylinders it sounds like he got away with it, they do sometimes bend con rods with a bad run away.