Unless they're current ATC or current airline, I would say they have no business on the flight deck of a commercial passenger flight
Completely agree. Thomson only allow current airline employees holding a valid airside pass into the FD during flight. Im lucky enough to be in that category so was/am allowed in.
They have to have a valid airside pass, as skipness pointed out with the BA example, even airline employees not on the front line can have hidden agendas.
Just because you hold a PPL doesnt mean you are automatically of good intention, even the 9/11 idiots took flying lessons.....