Looks like he did return the Mac Pro but not before dropping it down the stairs. Personally, I would have gone for the Nano than try and weasel Apple out of an $80 upgrade.The iPod and the Mac must be on the same receipt in order to qualify for the rebate.
If you haven't already opened the Mac Pro, just return it and the iPod, then have them re-sell it to you on the same receipt. If you have opened it, then you'll probably be better off just returning the iPod, as you're rebate will most likely be denied.
In this case, asking for a nano that should have been free isn't exactly being cheap. If the Apple Store messed up the receipts when he purchased what should have been a qualifying purchase, then it's the Store's fault, not his.Cheap people like yourself make retailers hate their jobs.