Judge operating system by familiarity (Mac vs. PC) nothing about a Mac makes it "good" for photo/video editing, and you pay 3x the price to fund their slick advertising campaigns and trendy packaging. Don't get me wrong, they make a solid product, but the software is key. (usually will be something from Adobe, mentioned above, and available universally on both platforms) If she's an Apple person, probably best to go that route for ease of use. If she's not, great! You can buy two systems, one for each of ya and still have money left to fill your humi!
If she has no preference and she's at all technical (good chance, seeing she's shooting beyond point and shoot "auto" mode) buy the PC. The biggest appeal to Apple products, whether the users will admit it or not, is that most of your choice and free will is removed. Apple decides what's best for you and THEY tell YOU what you want. To some, that's "easy" and "it just works". But again, she's not shooting a point and shoot, she's working with something with options and settings. To me, that's a great analogy: DSLR on auto vs manual = Apple vs PC