I moved your post. You don't say much about this trip, so I will speculate a bit. If you are as experienced as you say you are, why do you think this is a good idea?

I don't want to burst your bubble, but you are asking for trouble. A "very difficult three week" trip is no place for a self-taught beginner. I have no idea what you consider "very difficult" so you may want to elaborate. Backpacking isn't rocket science, but it isn't a trip to the mall, either.

A year is more than enough time to learn camping skills, but what you describe, including altitude, would cause me to rethink my plan.

I have first-hand experience at altitude (above 10K), including two hospital stays due to HAPE or pneumonia that almost killed me both times. I was fortunate, I wasn't camping and was in a big city with adequate medical care both times (same place).

Example-I flew into La Paz (13.5K ft.), got picked up and went home (11.5K ft.) Went out with a friend, had a couple of beers, went home and woke up coughing up blood. Woke up my parents who drove me immediately to a clinic. According to one of the doctors, another hour and I would have been dead. Had this happened in the outback somewhere, no way I would have survived.

Granted, not everyone will have the problems I had, but if your friend turns out to be someone who does, you are looking at serious consequences if she has a problem