Thank you so much for all of your responses, especially Mrs_Often - I showed her your response last night and she was very thankful (and impressed that it wasn't from genuine Kobo support!) although unfortunately she's still having a few issues with it, mostly the responsiveness.
I'm going to look for a case with a light built in, I think that's maybe the best way around the contrast issue, rather than having to keep changing etc (I think part of the problem is she was apprehensive about using something other than a book, and the default font is quite bookish, so she's reluctant to change, if that makes any sense)
I'll hold out for an update too and hope it becomes quicker/more responsive in the next, to be honest I find it to be a bit of a pain myself, but she's quite impatient with it and if it doesn't change within a few seconds she presses it again, which only seems to make matters worse! I think patience and perseverance is key here