Pilot Pete makes a very good point and a scenario which I would imagine could create debate. Is it safer at a critical time in flight ie DA of an ILS to 'let them go' than have a situation of conflict and confusion at such a time. There would be only so much you could do as PNF before you would have to accept that the PF has lost the plot and it could actually be safer letting him contimue and suffer the consequences later. These consequences hopefully only relate to paper work.