Don't they put a seal on the dip stick and oil fill cap? If not, anyone could take oil out so their engines would qualify for repair or replacement. Most dealerships don't allow customers in the service bays. It would be quite simple to overfill the crankcase, seal the cap and then show the owner the dipstick after he drove 2,000 mi. that the crankcase lost little or no oil. I believe that's already happened to someone here.
If the dealer is reputable.. . and an owner is that skeptical, I am sure you could ask to view the oil level OUTSIDE the service bay before any "seals" are placed on the dipstick or filler cap.
A simple request since it it the owners vehicle and also wants to make sure of any results. . . . before and during an oil consumption test.
Or. . if you don't trust the dealer. . . . change your oil, and sell the thing on Craigslist in "As-Is" condition.
There are always alternatives to any scenario.
And several here have just done an engine swap from a low mileage 2014, 2015 2.4L salvage vehicle engine which had upgraded pistons, rings, HPFP, etc.
All for $500 to $1,000 especially if done DIY.