My mention of ecosystem wasn't about being locked in. That really isn't much of an issue. Every ecosystem has plenty of books. The reason I mentioned it with respect to the Kindle is because it's ecosystem includes both ebooks and audiobooks,, although not on the same e-ink device.
Amazon has the Echo. Nobody else has anything like that. Amazon owns Audible.com and that gives them a big edge for someone with vision problems. And a smartphone can do the same thing as the Echo for listening to audiobooks or having the device read ebooks outloud and syncing with the Kindle.
I do agree that the Kobo might be a better device because of the text weight controls but those don't always work on sideloaded books. I've found them to be pretty iffy.
Another possibility to consider, is to convert the books for her in Calibre, including a bolder font and sending them to her via email. They'll automatically download to her Kindle and be available for her to read as long as you set her account up to allow that.
Converting books in Calibre and adding a bolder fonts typically takes 30 seconds so it's not really a big deal and it can also do bulk conversions very nicely.
Calibre is a free program. You can google it to find out more about it. Probably most of the participants here use it.
I live in a retirement home and I've helped a number of people here get started with both Kindles and Kobos and I've found that most are reluctant at first but if they want to read and if I'm patient most will learn how. There are always those who won't, of course. We have one woman here who loves to read and can't afford enough books to keep her happy and yet she refuses to try out the old Kobo I offered to give her. She won't even say why. If I mention it she walks away holding an arm out as if to ward off evil.
Barry