Not a physiologist, but my WAG is that the guy at high altitudes would be in more serious condition than the guy at sea level, in the same time frame.
All else being equal the guy at sea level will have a greater quantity of air (number of molecules) in his lungs because of the higher pressure, and thus a greater quantity of oxygen in his lungs. And the oxygen which *is* in his lungs will be at a greater partial pressure. If the blood continues to circulate, even though there is no new air being introduced to the lungs, the circulating blood will continue to be oxygenated to some degree to the point that the oxygen in the blood and the oxygen in the lungs reaches a physical equilibrium.
Compare that with someone suddenly subjected to very low atmospheric pressures. At one second before that, the blood is fairly oxygenated at normal pressures. I would speculate that immediately following the pressure drop, oxygen would be flowing out of the blood thru the lungs, so immediately blood oxygen levels would be dropping faster, sooner than the guy at sea level.
So: sea level guy, using up blood oxygen oxygen, but still able to utilize some of the residual oxygen in his lungs.
High altitude guy, blood oxygen being used up *and* being lost thru lungs from the start.
So, high altitude guy reaches whatever level of blood oxygen results in permanent damage sooner.
Not completely sure this is accurate, but that's my speculative opinion based on a little knowledge of chemistry and physics.
edit:
If you think about it, Time of useful consciousness at FL 400 is 15-20 seconds. Pretty much anybody can hold thier breath for 15-20 seconds without passing out. That seems to support my theory on the question.
Last edited by A Squared; 11th Aug 2015 at 08:07.