I read many of the posts here, and they mainly boil down to:
1 The dealer isn't required to give you a refund
2 The buyer isn't required to correct the form, but he should because it is the 'right thing to do'
My opinion? The dealer isn't required by law to make the customer whole, but why should the customer do the dealer a favor?
This is a problem endemic to all retail: They no longer care about the customer. This is why so many retail stores are going out of business. I am a diver as well as a gunnie, and many dive shops are suffering because large online stores are undercutting them. A brick and mortar store cannot compete in pricing with an e store, so the only thing they offer is customer service. Gun stores, with a government ordered monopoly, don't have to worry about customer service.
ME? I would offer a deal. I would bring the gun in for store credit in exchange for fixing the paperwork. If he doesn't want that- too bad. Tell it to the ATF.