I have a similar vision problem, although a different cause. I've tried many options and found that the Kindle keyboard, Kindle with Ads and Kindle touch were the best choices.
I also love my Simple Touch Nook, which is easier to hold, but slightly heavier, but I get annoyed at the shelving (it doesn't remove the book from the main list if you shelve it, so you don't know if you have all your Sci Fi in one place or not. If she keeps more than a couple of dozen books on her device, she will get annoyed with that. If, however, she is like my Mom and wouldn't sort the stuff into categories anyway, it is also a great option.
I have not tried the Nook glowlight and have a paperwhite on order for my Mom. More direct light helps, so I see this as a plus.
I have noticed over time that the text sizes available vary by the book, not the device. I've had some books that I just zapped off because the largest type size wasn't large enough. I wish I knew how to fix this. Kindle is a little easier to use, IMHO, for adjusting type size than the Nook, but not actually very hard for either. I love the touch screen, but some people have reported that if they have very cold hands (common among elderly) that they can't get their touch screen to work properly.
The higher end ereader devices (like Kindle Fire, etc) are fine for someone with good vision. All those doodads aren't useful to those with bad vision because the text is too small to work with. So, your best bet is to stick with something that focuses on READING and not internet use.
Note: the Sony and Ipad both have too much 'shine' making reading difficult. You can get non-glare covers, but I found that this also reduced the contrast.