If you inhale nitrous directly from a nitrous tank (and not putting it into a balloon) it could happen. I don't really see this happening any other way. If he was just using nitrous cartridges that you order online, I really don't see a way that this could happen because you either need to crack it into a balloon or buy one of those whipped cream cans and crack it into that and then inhale it and is impossible to freeze your lungs that way.