flyremnant
The following are from ICAO Anex 2 which may help, however you probably need to look at how your own governing authority views it.
2.3.1 Responsibility of pilot-in-command
The pilot-in-command of an aircraft shall, whether manipulating the controls or not, be responsible for the operation of the aircraft in accordance with the rules of the air, except that the pilot-in-command may depart from these rules in circum-stances that render such departure absolutely necessary in the interests of safety.
2.4 Authority of pilot-in-command of an aircraft
The pilot-in-command of an aircraft shall have final authority as to the disposition of the aircraft while in command.
Indeed it is important to recognize that the flight deck is not a democracy, by law.
There are a number of different approaches that airlines have taken to their CRM training, all of which require the second in command to speak up and perhaps even take action to prevent an accident. An example is UAL's CUS words. There is also a model called PACE by an American Airlines Captain. We have used the latter in CRM training to a good response.....Google PACE and you will find it.
Hope this helps.