17" will be a bit heavy but doable if she uses a backpack or shoulder bag to carry it. I usually stick with the 15.6-16" laptops because the smaller ones, like Nate said about, are just too close to a tablet (a real tablet, not these 7" e-readers) to make them worthwhile if you already have a tablet.
Lightweight with ssd, powerful cpu, etc, I would look at the ultrabooks. Massively overpriced IMHO but if you want to keep the weight down, they will do just that.
I would also suggest a laptop with 2 drive bays in it so you can have the ssd for booting and some apps while you could have a standard HD for storing any data she would need to keep, pictures, videos, etc.
The screen size is going to be the biggest kicker in how many options you have as 17" screens are less prevalent than in the 15-16" range, mainly due to weight.
How many people think computer users are a bunch of wussies? LOL If you look at a laptop and say, "WOW, that thing weighs 5 pounds" and think that is heavy, we need to get you on an exercise program ROFLMAO I know carrying them in your arms all day does get heavy but that is why they make backpacks and shoulder bags (cross shoulder bags are the bees knees)