It is worth remembering that not everyone is a seasoned air traveler and there is no user manual that the novice traveler is required to read in advance of arriving for their flight.
Lacking knowledge of the dangers of airside operations, or the general security paranoia of all airports, one only needs to add the ingredient of panic at seeing one's supposed aircraft departing to understand how this gentleman may have behaved the way he did.
The responsibility is entirely with the airport to ensure that kind of behaviour is prevented. If there is no aircraft at one end of an air bridge then there should be a locked door at the other end.