Librera. I found it daunting at first launch, but gave it a chance and once set up it's perfect. You could set it up for her and it would be as friendly as it gets.

For example, my usage now only ever goes like this:

1- Open Librera. Presented with a scrollable page of all book covers.

2- Scroll down to the epub I'm currently reading. Tap it once.

3- Resumes the book where I left off, with previous view settings. In my case full screen text with zero UI (tap once to hide the UI).