As someone who used primarily windows for 20+ years, used the surface pro line from the 1st through 7th iterations, and now only runs with a MacBook Air, I would say absolutely get the cheapest M1 Air you can find.
If she’s not a heavy user and is just doing word processing and light computing, it will be vastly more than enough machine, and be best in class in the most important things I can think of when thinking about a laptop for a writer:
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nice big, bright, high res display for crisp text and easy reading/writing
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insane amounts of battery life for writing in cafes or libraries or wherever for probably longer than she could stand to work in any given day
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nicely portable and discreet for carrying around to places where she can write
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decent keyboard and possibly the best laptop trackpad around for ergonomics (such as they are, in a laptop)
Getting used to MacOS will take a few days at most, and there are plenty of free/Foss apps to improve quality of life for Windows users moving to macs.
Last I checked, Kobo will be better specs (screen, water proofness and connectivity) for the money, and if you’re technical it can be modified very heavily, including pretty easily user expandable storage.
Kindle will have a more seamless Amazon experience and maybe better support.
I have a Kobo Clara HD, and I love it to bits. Warm temperature backlight, and I have installed custom firmware on it which lets me use a different reader app, and run an SSH server on it so I can remotely transfer files etc.