Best software for a beginner - Photoshop Elements - whichever the latest version is. Or if you want to be very cheap you can get GIMP, which is totally free and more powerful, but has a less intuative interface - it can do stuff, but getting it to do it is a challenge so I hear.
I'm not dumbing down editing in the least and there are some things for elements (like layermasks and curves) that you either have to live with reduced functionality or find a free addon for. However at a silly £5-600 odd CS5 is just far too much money for a beginner to invest in. If they do have that sort of money I'd suggest a few addons to their photography gear instead - lenses, tripods, flashes, lighting even a days workshop - work on capturing the photos first.
You can always edit a photo later - but you can never go back to capture a scene in the past.