my suspicion ... and why i asked the questions about the file ... is that the size of the file does affect battery drain. i have an 8MB epub I sideloaded, and I noticed incredibly fast drain on my battery. I've had a couple other kepubs that have been missing some of the chapter anchors and so they load large chunks -- 150 pages or so -- and they seem to drain the battery a lot faster than normal sideloaded epubs.
so, in my experience the size of the file, and how much of it gets loaded at a time, seem to affect drain, which I think is what Mrs. Often is also noticing on her device.
personally, I always power down rather than put in sleep mode, unless I know I'm returning very shortly (like after fixing a cuppa). and I always return to the home screen. i don't know that either of these really change the consumption, but if there were a way to minimise battery consumption when not reading, then turning it off, or else putting it to sleep at the home screen (rather than with a book loaded) should minimise the drain.
i've also just had a week where the battery meter kept bouncing between 90 and 100 ... and it sorted itself. so hopefully this was a one-off flakiness that's been sorted so she doesn't have to replace the battery.
in the Netherlands, do you have battery stores that will rebuild internal batteries? I've seen them occasionally ... stores like this: www.mdsbattery.co.uk/
i've had such a place rebuild a laptopn battery before for half the price of a new battery, and it worked fine.