The kindles are light as anything and really good to bring around. You can get the type with no keyboard and they are really conveinient. That's what I use. Wife these days just uses either her tablet or ipad. She got a good deal on a 3rd gen ipad a couple weeks ago. It's nice. Anyways, the tablet/ipad is heavier and has a backlit screen so not as easy on the eyes.
Kindle (the old fashioned kind) is not lit at all... it reads like a page in a book. If you are reading it in the dark you will need to have a light on, or you will need a little case type thing you can get for it with a built in light. It takes power from the battery. The little kindles batteries last for EVER. It's great, really light and I love it.
I wouldn't mess with a Kindle Fire or another like it. I'd just go with a tablet/ ipad.
Tablet/ipad, heavy, backlit (hard on eyes), not easy to see in the sun (kindle is great for this, no glare)... however, it does anything a computer would do. So my wife likes to read her ebooks, then check facebook, RIU, etc.
If you are getting an Ipad, get a kindle too, they are cheap in comparison. Then she'll definitely be happy lol
EDIT: Amazon Kindle $90 170 grams
Ipad mini $330-660 308 grams