Ignorantia juris non excusator ignorantia legis neminem excusat (Latin for "ignorance of the law excuses not" and "ignorance of law excuses no one" respectively) is a legal principle holding that a person who is unaware of a law may not escape liability for violating that law merely because one was unaware of its content. European-law countries with a tradition of Roman law may also use an expression from Aristotle translated into Latin: nemo censetur ignorare legem (nobody is thought to be ignorant of the law) or ignorantia iuris nocet (not knowing the law is harmful). His possession in New York or New Jersey is illegal. How long has the gun been in the metropolitan area and how could he have no idea of the consequences?
Yup. That's exactly where his biggest problem is. The "I did not know" BS might work for the airport part but not for the possession in NYC one.
It doesn't matter about criminal intent. He should have known better. Anyone who owns a gun knows each state is different regarding gun laws. The correct way would have been telling the Delta check-in person that you have a gun (no bullets). They would have tagged the bag firearm in the bag. I've done it before at Laguardia Airport.