You can list as a 'cabin only' jumpseater from Mexico, however, this is confusing to the gate agents, especially if their english isn't great, because they're hearing "jumpseat". The same probably goes for a lot of captains. Someone says "jumpseat" and they don't think it can be done. Often, you've got to go up there and explain the situation. I've had cabin jumpseaters a few times from Mexico, they were with other airlines though. But yeah, sounds like this guy thought he could sit in the actual jump, which is surprising.