You can get Lightroom relatively cheap ($80), and if all your wife is doing is managing and touching up photos instead of manipulating them, Lightroom is perfect. LINK
The full name for Lightroom is “Adobe Photoshop Lighroom”, which may sound confusing, because it contains the word “Photoshop”. In a way, it makes sense, because Lightroom can be considered a subset of Photoshop with specific functionality that Photoshop does not and probably will never have. It was created for the main purpose of managing a large number of images, keeping them organized in one place. Photoshop is a very advanced image editing tool, but when you edit hundreds of images, keeping them organized becomes a problem over time.
gimp is good, but it doesn't support .raw files (I don't think), which, if she's wanting to edit her photos most effectively and realistically, will need a program that does.