I think the rules should be changed in this day age of heightened security and passenger safety risks.
I dont think anyone other than the designated pilots should be seated in the cockpit during a flight unless they are immediate staff members and have been requested to by the pilots for some safety reason or other.
I remember the thrill myself of visiting the cockpit, I was six years old at the time.
Things have changed since then but not obviously for Cathay. I also dont believe it should be left to the discretion of anyone when and who can sit in the jump seat in the cockpit, not the pilots nor the check in staff. There should be strict rules governing that, yes absolutely and it shouldn't include immediate family. They can as easily sit in the galley jump seats.
This was short haul remember, the pilot shouldn't have to be the one instructing a passenger on the use of filming equipment they have enough to be doing. I expect if its is indeed a relative its a relative of the ground crew who might not be so versed in the rules.
I do feel sorry for her she has unwittingly opened a can of worms, she also looks (if she is indeed an employee of Cathay) to have just started her job.