His problem there is two fold: (1) Ignorance of the law is no excuse, (2) as a weapon's holder he's charged with the responsibility of either knowing or looking up New York's laws before he brings a gun there. Whether that's fair or not, that's basically how its treated. If you cross into another state, you're responsible for their laws. End of story. Some state's reciprocate with permits, but not all do or have an agreement with all state's.
We had a similar case where a guy brought in prohibited weapons (knuckles) that were permitted in Kentucky (with a permit), but are absolutely prohibited here. We reciprocate with Kentucky, but only with respect to certain firearms.