There's a very old saying: when in Rome, do as the Romans. Might have helped this guy to know what the rules were where he was traveling to.
There are enough restrictions in the USA. He would have certainly been prosecuted domestically be it a slap on the hand or the possibility of jail time for his actions. In the USA you do not carry a gun on an aircraft W/O special privileges. A possible criminal with intent or an idiot who does not have carry permission on aircraft are the only two possibility domestically!
This is way off the board to contain a restricted firearm into a foreign country in your carry on luggage, ESP not declaring it. If he declared it and it was not a restricted firearm his position would be much better, if he declared a restricted firearm out of ignorance it sounds like Canada would probably be much more accepting of idiocy.