Sometimes the full fledged Photoshop has WAY more features a normal user will ever use. Check into Photoshop Elements, which is what I have been using. It is much cheaper than full Photoshop and has the major components that the average person would use. It is similar to the full version enough where if you learn the techniques on Elements, you can always move up if she needs more features in the future.
I have learned that the hardest thing about photo editing is just finding the time to sit down and do it. There are many YouTube videos out there to show how to use the software as well as other tutorial websites.
One thing to note, if you want to do any sort of real digital photo editing, make sure her camera takes photos in RAW mode so you have the full power of lossless images. JPEG images can be hard to edit due to already being compressed once.
Also some may also mention GIMP for editing. It is powerful, but not as user friendly. If your wife is a techie then she'd be fine with it. If not, stick with the Photoshop lines.