I use mine solely for PDF/CBZ use-cases, and far prefer the Sage for novels, FWIW.
The main strike against it is the weight, so if that's a non-issue, good for you

.
The other possibly not inconsequential one is the lack of warm LEDs: it's... less of an issue for comics, but I would possibly be annoyed at the lack if I were reading novels on it, as I tend to prefer a warmer frontlight than the Elipsa's fairly harsh blue.
I concur that battery life is mostly not an issue on it, mostly thanks to its gigantic double-cells

. (As long as you don't use the pen, pen drawing murder batteries).
If she's happy with the form-factor of the Forma, I would possibly not rock the boat and go with a Sage instead? But if you feel like the extra screen size would help, sure, why not.
(Keep in mind the Elipsa screen is *lower* density than the Forma/Sage, which is a strike against it for novel reading in my book, but is admittedly a fairly moot point at large font sizes).