Well it gets a bit circular at that point. They can't act as a flight crew member because they don't have a license or rating !
You need to look for something in the legislation which forbids a non-crew member from touching the controls (as there is in the case of commercial flights), or something which describes the pilot in command's responsibilities as sole manipulator of the controls.
I can't find anything.
The ANO defines Flight Crew as:
in relation to an aircraft means those members of the crew of the aircraft who respectively
undertake to act as pilot, flight navigator, flight engineer, and flight radio telephony operator of the aircraft.
It then fails to define Pilot !
It does define Pilot in Command as:
means a person who for the time being is in charge of the piloting of an aircraft without being under the direction of any other pilot in the aircraft.
So your pax is clearly under the command of another pilot in the aircraft and is therefore not Pilot in Command.