Any passenger is just a passenger pure and simple.
I have not read the accident report deliberately so this reply is not biased by that - Yes the pilot in command is at the end of the day the person in charge - even if he is a new PPL and the passenger is a 20,000 hour Instructor/ATPL, but I would not go along with the above quote -
any pilot who is flying as a passenger and sees something not right must make his concerns known, this can even be as an airline passenger alerting the flight deck via the cabin crew that there is for example a build up of ice on the wings. In the case of sitting next to a PPL in a deteriorating situation I would be raising questions such as "are you sure you are happy with this situation" or a statement of " I am not happy with what is happening - are you sure you would not like me to take control", As the situation gets worse I may eventually get to the stage where I
would try to take control - but that would only be when I reached the stage where I was convinced the pilot flying was going to kill me if I did not! At the end of the day though, it is the
designated pilot in command who is responsible for what happens.