Haha just went through this at my house Buddy. Girlfriend was wanting a e-reader. Did a little investigating these and actually went for a Toshiba Thrive tablet instead. These are 7 inch and the Thrive was 10.1 inch. So why the Thrive and not these.....
Well these are both actually android tablets. The Nook is limited in it's abilities running froyo and the Fire is a full android tablet I believe running gingerbread. But both being pushed as an e-reader they were limited to everything they could do. I know she said just a reader but if you got these she might want to do something else since it is available and I would be worried that she might run into issues.
So I did some investigating with the tablets and the thrive is the same shape but a little bigger. It is running Honeycomb. It can do exactly everything these 2 readers can do and more. Replaceable battery, many ports (more ports like a USB that other tablets, readers, and even the ipad doesn't offer), faster processor and more RAM. Very good quality too and gets great reviews. The only thing I really did not look into was how each looked in direct sunlight.
But the biggest winner for me was price. I picked up the 8GB tablet which normally sells for 379, on black friday from newegg.com for only 199. Win, Win!
If you are still interested in the other 2 I would see how they look in direct sunlight. See what kind of touchscreen they have. Also check where and how you can download books and movies from as these 2 may have other downloads blocked. Also check on battery life. As I said the Nook is running froyo and limited in its abilities so that would be the biggest downfall of that one. Hopefully I helped you a little bit.
Review of the Fire with video.http://www.techradar.com/reviews/pc-mac/tablets/amazon-kindle-fire-1041946/reviewhttp://reviews.cnet.com/tablets/amazon-kindle-fire/4505-3126_7-35022491.html
Review of the Nook with videohttp://reviews.cnet.com/nook-color-review
Review of the Thrive with videohttp://reviews.cnet.com/tablets/toshiba-thrive-16gb/4505-3126_7-34468401.html
Make sure you check the reviews about the android market being removed and no cameras.