I'm not going to offer very much help here; I don't have the requisite experience in the conditions the OP describes for the hike.

However, I would like to elaborate on two of Gershon's points.

First, I agree about walking every day, regardless of conditions. That will mean she will walk in the rain and/or snow - and on a 3-week trip, it's more likely than not that you'll have to deal with foul weather. This will get her used to that - and also give her a chance to make sure her rain gear is up to the task. (Just remember that heat retention characteristics may be different where she's living than they are where you'll be hiking.)

Second, if she's going to learn the camping skills remotely, she'll have no choice but carrying her own complete set of gear - you're not there to share with her. If you plan on sharing later, she may be able to rent gear for the practice trips; however, depending on how many practice trips, and whether she intends to solo after your joint trip, it may be more cost effective to buy her own gear. I don't hink it would be necessary to get the lightest and best (though she may want to); something reasonably light might actually be better, since it would result in perhaps a slightly heavier pack than she'd actually be carrying (thus do a better conditioning job.)

Good luck; I think you are taking a big, big bite - but I don't know enough about either of you, or where you are going, to say that with any authority.