What about a 3-crew operation, or a check pilot on the jump seat. Can they call "Stop"? If maybe, is that a seniority-based command for the bod on the jump-seat?
Many of us fly 3-crew ops, sometimes there is even a 4th pilot on the flight deck, and they may spot something seriously wrong before the others.
Surely this is a common-sense issue. Anyone who see sit, calls it. And the consequences of a "Stop" call are the same for all.