Any warpage to the rotor at all I'm sure he would have felt. Anything significant enough to cause rapid wear to the pads probably would result in a tow-in situation-intolerable shaking.
To me it's strange that there wasn't a heavy accumulation of brake dust-something doesn't make sense here.
Still, I wouldn't let the dealer get away with charging him for it, that just bothers me. Most wear items are covered for a year or 12,000 miles, as long as there is no evidence of vehicle abuse, and a good set of rear tires is evidence enough of no vehicle abuse. How else they gonna prove vehicle abuse resulting in worn out rear pads? I would THINK someone at MB would be interested in this problem, but don't know how you would get there interest. Someone at the dealer should call the MB TAC about it.