Prior to purchasing my Hustler, I ran across a Fountain 35' Lightning in the Philly area at a boat manufacturer. This boat had a pair of new 502's in it, and there was a receipt for the work. When I called the shop in southern PA that did the work, they confirmed that they had in fact did the work. However, when I asked them if they were Mercruiser crate motors or GM crate motors, they said they were GM crate motors. They said they got them from a Chevy dealer, and had done this a number of times for boats. I asked if they changed oil pan, cam, valve spring,etc. They said no! Just dropped 'em in! I was very surpised as they claimed to specialize in auto & marine performance. This shop said they never had any problems with just dropping in the GM crate motors. But, not what I'd want in my boat without proper upgrades. As outfit that had the boat wasn't willing to flip the bill to correct it, I passed on the boat.
Unfortunately, it sounds as though your friend found out one of the areas that are different from the GM crate vs Mercruiser: cam & valve springs. This also results in a different power curve between the GM crate & Mercruiser engine. However, the GM crate motor can be upgraded to the Mercruiser spec's, so it's not a situation that a few $'s can't fix. Unless something was trashed when had valve issue, should be able to salvage the existing crate motor.
Also the GM crate 502's are black, and typ. will have a GM bar code sticker on a valve cover. But, as Cord stated, I wouldn't base this on the color of the engine oil pan.