I bought my first e-ink tablet, a remarkable, to read academic PDFs and was disappointed by the software and by the low contrast of the screen. (I fell in love with the rM as a digital notepad, something I didn’t know I wanted until I had one, but that’s another story.)
Later I got a ten inch Boox and it’s an amazing PDF reader. It has a frontlight so the contrast is boosted, and the “article mode” (google Boox article mode for a short video) makes multicolumn PDFs so easy to read. (Unicolumn still need to be read in landscape mode.). I wouldn’t trade it for a larger model because the article mode is so good and I like the portability and handling of the ten inch model.
She can try an rM at Best Buy if you live in the States. rM is also plenty Apple compatible, since it has both iPhone and Mac apps. But it sounds like maybe she just wants an iPad?