Thats a bad place to be. In some parts of the world the captin is like god where FO can't say anything. Remeber the Korean Air B747 crush is Guam. The captin flew glide slope that was officially inop. FO asked the captin please lets not fly it down. Than when the GPWS went off the FO said again please go around.
Dumb ass captin flew the whole airplane into trees. But yea if I'm FO I won't say anything as long as my captin won't kill me. If I see he will fly me into a mountain or something I'm punching him on the face if I have.
So my opinion captin must be stopped with no hesitation in situations like flying below DH or MDA with no vis or flying a non operative approach, or flying a vector toward a mountin.