Font size is sticky for book text. You can enlarge or shrink text with the two finger pinch or zoom gesture, or you can use the "Aa" menu which you reach by tapping the top edge of the screen. Font size settings don't apply to the device menus or book lists, so if the text in the book list is too small for her it may be hard for her to switch to a new book. There is a built in Screen Reader that provides some audio help with navigation for the visually impaired, but I have not tried it.
Getting books onto the Kindle and choosing a new book to read may be difficult if she's not comfortable with tech. It depends on whether she'd be buying books from Amazon or borrowing from the library via Overdrive or if you'd be the one setting up her e-book library. The Sort/Recent setting on the book list would help her find books that were newly added or those that she'd just been reading.