Did you see the car drive in when returned / did you start car soon after it was returned?

Any idea if he used Bio / veg or other non standard diesel?

Wrong type of oil: Not a problem even if filled with gas oil in the short term. ( As in 10's of K miles )

How many miles does this car get per Qt and under what conditions? ( round town sub 40 MPH will use less oil per mile due to low loads , highway 60 + will use more due to higher speeds / loads. ) How many OT does this car hold?

No start due to out of oil: If the engine is cranking at normal speed, not an oil problem. The first parts of an engine to suffer oiling deficiency / locking up will be the connecting rod bearings, next will be main bearings. Pistons / rings / valve train won't going to be a problem.