There was a vintage of Series 60 engines that had problems with the pistons separating at the wrist pin. From my understanding defective pistons from a supplier were the problem and at the time this was occurring Detroit would rebuild or replace the engine.

The problem as described to me would show up in the early life of the engine. If the engine survived past 250,000 miles it was unlikely it had defective pistons.

I don't know from the description if that engine failure was the result of a defective piston, but it is worth trying to find out. The owner should go direct to Prevost to try to enlist their support in determining if that was the cause and if it was to influence Detroit to stand behind the repairs.