How do you treat altitude sickness?

Depends on degree: This depends on the degree of illness. But I do agree with my colleague that descent -- safely -- is the first thing to do. Additional needs might be oxygen, hydration, the diuretic diamox (acetazolamide). If the person is confused, short of breath, and not moving well, this is severe and you're going to need help getting him/her down the mountain. Better to watch for early signs than let it get this far.