Earlier quoted context omitted.
You don't have to pretend to be a pilot. Any cabin crew is allowed in the cockpit, AFAIK
Not just cabin crew, a lot of the time anyone flying standby is offered the jumpseat if there are no other seats available out of courtesy. Especially if they are an airline employee, but often non-employees too.
I have never heard of a plain clothes non-employee in cockpict jumpseat.