"best" is subjective. for me, a graphic designer and retoucher photoshop is the most useful, but for someone who doesn't know retouching or photoshop extensively, it's overkill. most people who "use" photoshop probably only explore about 2% of its features.
If she wants to make flyers and things like that, Adobe Illustrator is fantastic. It's not a photo-editor like Photoshop, but it really is a great program to use.
Everything your describing she wants to do screams Photoshop Elements. It's great for organizing all your pics, doing any editing required from the most simple to very complex, and has a lot of great out of the box stuff for making flyers, invitations, cards, etc.