More to the point was how did the person arrive at the altitude situation, and how high is he/she...
At near space, and explosive decompression then good bye..
The decompression chamber example, is at a height above 10000 ft usually. At that situation, there are several minutes before hypoxia sets in. (very similar to intoxication) As the brain runs out of O2, disorientation sets in and eventually collapse. Application of O2 mask results in rapid recovery. I have done this a couple of times in a RAAF Decompression chamber.
In neither situation would Expired Air Resuscitation be of any use as the "Rescuer" would most likely be affected as well.
Being at sea level physically but in the chamber at altitude, is the same as being at the actual height.