Two thoughts:

She needs to start hiking a lot, like, every weekend. A good part of that needs to be at higher elevations.

She also needs to get herself educated - wilderness medicine basics, navigation, etc. And, also, she needs to go on a backpacking trip of a week - after working up to it. Not everyone is able to withstand isolation like you're talking about day after day. I've had people freak out on day three and want to go home, despite a number of years of one and two night trips. Some people just cannot backpack that long. So she needs to know how she will react to long backpacking trips. Not car camping, not overnight jaunts.

She doesn't need to trust you - she must trust herself to be alone in the situation, and she needs to be honest with herself about that, too. The last thing either of you needs is what will happen if she goes and she is not ready for such a trip.
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"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

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