The reply was "in this country we don't have such a law". I did remind the guy that the aircraft was not registered in his country, so in theory, the laws of his country don't apply anymore, it's the registration that counts, he has to adapt.
The argument is completely moot. When that person bought the flight ticket, they agreed to terms of carriage of your airline, which most likely includes a clause that the passenger will obey instructions from the crew, particularly those related to safety of the aircraft and its occupants. Should they fail to do so, you have a very good case for denying them further travel. Aircraft current location and country of registration don't make any difference.
Glad you got it sorted out in a friendly manner, though.