That's a possible scenario but a 1/2 mile hill at 5-10%, even if only the rear brake was used, shouldn't have overheated the rim to that extent. If you were describing a 5 mile, 10% downhill in the Alps, maybe.
i think it's most likely he had the tube partially pinched due to improper installation before the blowout and possibly the minor stress of the decent was the last straw.