@aviatordom
..and does the same card tell you how to descend from a door that is 7-9 m (guess) off the ground ?
I'm sure they didn't leave it connected to an air-bridge.
As for
"the crew could have done a final check of the cabin before leaving, but.. "
I don't think it's a matter of "could", I am sure that they are mandated to do so.
I hold no brief for people who take no responsibility for themselves, but I cannot see any way in which the cabin crew were not grossly negligent in their duties in overlooking a passenger on-board.