Whilst exciting for the person involved, this is totally unacceptable and against HK CAD regulation and CX company policy. Visitors are only allowed when aircraft is parked on ground. Under no circumstances can a passenger ride in a jump seat or be permitted in the cockpit at any time during flight - unless they are authorized to sit in a jump seat e.g. deadheading crew.
I mean anyone can pose as an aviation enthusiast and pretend to be interested in the aircraft.... The Jump seat is behind flying pilots. I would feel very uncomfortable as a pilot or passenger knowing there was a complete stranger sat directly behind one of the pilots. There is a reason this rule exists, and actually some rules are there NOT to be broken, this is one of them.
Whilst I would not want someone to lose their job over this, I'd want them to at least be talked to for this should not happen ever.
I take this kind of thing very seriously. Cockpits should never be compromised. Not to add wild speculation, but who knows if a passenger had been let inside the MH370 cockpit?? The point is, to ensure a safe and sterile environment cockpit should never have passengers inside or near during flight. Its perfectly fine though, when the plane is on the ground.