Actually it is perfectly clear cut.
The F/O is authorised, indeed required, to take whatever action is necessary to ensure the safety of the aircraft.
But, and it's a big BUT....
He has to be able to justify his action at the subsequent Inquiry.
He needs to be crystal clear on the grounds before taking the action.
Get it wrong and he loses his job and may be prosecuted BUT.... most pilots who have been there would say it is better to be alive and unemployed rather than the alternative.
As with everything in aviation, it helps to have some of this thought through before it happens.