Netbook

First off, you should know that later versions of WoW, such as wrath of the lich king have been optimized for multicore computers. The graphics are still run pretty easily by a modest graphics card. If he insists on getting a netbook (which I wouldn't for any type of gaming), then I would steer him toward one the newer ones with an AMD dual core. Single core will not work well for WoW.

Looking at the options, it looks like a netbook approaching gaming levels of performance will be in the $400-500 range. At which point, he would be better off getting an entry level 15" lappy as you would gain CPU performance in exchange for size.
Netbooks are neat little devices, but nobody games on them. They are meant for long battery life and mobility, not as desktop replacement. They even struggle with most multimedia, and usually need a separate dvd drive.

Here is an example of one of the netbooks I was talking about. From the reviews, you can see where the slow CPU speed is a bottleneck for the other components:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Here is examples of the entry level laptops I mentioned that are better equipped for gaming:
http://www.newegg.com/Product/ProductList.aspx?Submit=E...

Bottom line is for gaming with Wow, I think anyone would be disappointed with the performance a netbook supplies.

Pretty sure your friend meant notebook. Link the one he is looking into and people can give you their opinion.

Here are a couple options in the $350 range.
Pentium P6100 2.0Ghz & Intel HD Graphics or AMD E-350 1.6Ghz & Radeon HD 6310 graphics

You can find the "minimum" & "recommended" computer requirements for any given game on the box or other advertisements for that game. I believe trying to run any modern game on a netbook is impossible.