Definition of PiC is contained in article 155 -
Interpretation
‘Pilot in command’ in relation to an aircraft means a person who for the time being is in charge of the piloting of the aircraft without being under the direction of any other pilot in the aircraft;
Now - Although I don't know of anywhere where it states that the PiC is the final arbiter of how the aircraft is operated, various entries in the ANO / Rules of the Air state that the Commander / Pilot in Command is responsible for ensuring that the flight is conducted in accordance with the above documents. With that in mind the PiC must be the final authority to ensure that compliance is adhered to. It also means of course that they are in trouble if it is not, (unless it can be justified from a safety point of view).