Reddit reviews Backpacker Long Trails: Mastering the Art of the Thru-Hike

Does she hike much? If not, definitely do some day hikes with similar mileage and elevation gain to your planned trip so that she can make sure her sock and shoe combo works for her regarding blisters and such.

I feel like getting bad blisters could be a major bummer on a fairly long trip like that. Most other stuff can be conquered by a good attitude and snacks, but blisters on day 1 of 5 would be no fun.

You might find that either you reading a book written for beginners to remind yourself of what beginners need to know, or her reading one (depending on her preference) might help. Something like Allen and Mike's Really Cool Backpacking Book or Ultralight Backpackin' Tips or Long Trails by Liz Thomas might be a solid choice.

And of course, ask her if she has any concerns, what she's excited about, etc etc.

Regarding buying gear, don't forget that if you buy something used, and don't damage it, you can probably sell it for pretty much the same price you paid for it. So keep an eye out on /r/ulgeartrade and similar forums.

Here's the Amazon link to the book.

Novel:
https://www.amazon.com/Thru-Hiking-Will-Break-Your-Heart-ebook/dp/B00W4RICAM

Informational:
https://www.amazon.com/Backpacker-Long-Trails-Mastering-Thru-Hike/dp/1493028723/