I would say as a baseline, if she's mainly going to be doing things like checking email and doing papers, web browsing and music, etc but not a lot of high power stuff, anything with a dual core and 4gb of ram or better is going to be fine. I personally am an acer fan.
We bought my wife an acer aspire 5536 for Christmas of 2009. Laptop is a workhorse. It was 399.99 at the time of purchase. Has an AMD Athlon Dual core 2.2 ghz(I think) processor, 4gb ram, 320 gb hard drive. That computer is almost NEVER turned off. She usually lets it sleep, I've never had to open the machine. It is fairly quick, she is constantly online, on facebook, watching youtube videos, listening to music, doing bookwork, she is always using it. Nearly zero problems. I think I may need to clean the dvd drive out, as it had a problem reading a cd last time I tried, but other than that, and needing a new battery, laptop has been great. For a machine that is 3 years old and has Windows 7, it runs like a top.
Best buy may have a good deal, but I prefer Microcenter if you have one nearby. Many times they have decent Acer and ASUS laptops in the 300-400 range, though they do have higher end units. I would also strongly consider their warranty. Many of their parts, I can buy a 2 year walk in warranty, and if I have any problems, the receipts are on file in their system, I take the parts back in, no questions asked. Tell them what's going on, get it replaced or credit toward another item. Now I would say a laptop may be more scrutiny, but this is the experience I've had when exchanging just parts.
They have this laptop in stock now
http
/www.microcenter.com/single_product_results.phtml?product_id=0382601
They seem to always have these in stock. But the full number pads also make them great. In 2 years I've only had one acer come in for repairs(hard drive failure), other than that, mostly hp's, dells, etc.