Presuming there's no actual problems and it's never used oil like that previously...

If you change the oil the motor will need to have x-quarts of oil put back in. If you change the oil AND filter it will take a bit more oil to refill it.

Perhaps he neglected to add the extra for the filter.

A long shot, but that's all I can think of unless he just didn't put enough in it in the first place.