I have the 2011 edition and love it, but am not sure about the differences between 2010 and 2011 (likely they're virtually identical - according to Amazon, they even have the same number of pages). In that case, she might as well go with whichever is cheaper.
As for where to buy it, I'd recommend Amazon as long as she buys it in like-new condition! A lot of the sellers are trying to get rid of books with the answers already circled, which isn't worth the money she will save - it'll be annoying at least and will probably bias some of her answer choices.
Hope this helps!