Lemme go step-by-step to explain where he went wrong.
Father died, left behind his Rossi revolver. Presumably, it was properly licensed, which means he had a county-issued permit that listed the exact firearm in question. Any handgun must be individually listed on your permit, and must be transferred through an FFL, with a NICS check, whereupon your county sheriff's office will issue you an "amendment" to your permit listing the new gun.
So no, if it was a shotgun or rifle, there would have been no problem. Not because of "omg ebil gun", but because that's the permitting law. Shotguns and rifles may be transferred without paperwork, individual-to-individual.
Now, when this dude inherited the gun, he had a few options:
(1) He could turn in the firearm at his sheriff's office, free-of-charge and without penalty. He could also have called, and they'd have come and got it.
(2) If he had a permit, he could have transferred it to himself through an FFL. Here's the Oneida County Office of Pistol Permitting's handy-dandy little "Transfer From a Deceased Owner" sheet.
https://ocgov.net/oneida/sites/default/files/pistol/TrnsfrOwnership.pdf
Literally, unless you need the firearm to be disposed from the estate, they just want you to go to a dealer. Easy-peasy. Fee would probably be between $25 and $50.
(3) He'd do #2, but he'd need to get a permit first. This presumes he could pass a NICS check, but otherwise, New York is a shall-issue state. The cost for this would depend on the county in question. Oh look! The Oneida County Sheriff has another sheet!
Application Information | ocgov.net
So he'd need the classroom portion of NRA Basic Pistol, plus the transfer fee for the FFL. Not a big deal.
Unfortunately, this dude didn't do any of these things. It sounds like he was maybe not in the greatest shape to begin with, living in a house with no water or electricity. And frankly, all the media hoo-hah over those "illegal guns" just scares people. Most guns inherited this way are probably unregistered, just because people are too scared to go through the process, when most sheriffs honestly couldn't care less and are very happy to help.
So--no sympathy, but whatever. Sounds like the prosecutor agreed that this wasn't worth the people's time.