I use IrfanView for all the quick and dirt things. Sometimes I need a quick fix and crop, that's what I use. Small, free and good. No layers and magic wands or shaped marquee tools. That's the one that's not made for a Mac.
Otherwise with some older cameras Elements was included, and it's surprising how much it does. Elements 2 was the last version that ran on Win98, and I still have some older computers using that.
Elements 5 was the last version I had before finding Photoshop 6 on one old desktop, and Corel 5 on a laptop. (I have many hand me down computers) But I stuck with Elements for photography.
Finally I picked up Light Room which I have and don't know how to use. It's supposed to be the tool for photographers and doesn't have all the extras that CS3 (soon to be CS4) has built in, that most of us will never need or use. Light Room handles Raw, organizes files and does non-destructive "developing" of images. So the original is never altered.
With that. As long as she's happy, and it does what she wants, that's the best software for anyone. Don't bother with what's best for me, or someone else, it's always an individual decision.
I always thought of GIMP as Linux based, and since you can see from the above that I'm already confused, I never tried it.