Just found these cool reflective eyes on Ebay for $7. Perfect for the back of your helmet, eh? :)
Everyone needs motorcycle stocking stuffers! Most motorcyclists I know love the idea of being more visible in one way or another. If she doesn't want to wear these on her helmet, she could probably add them to her jacket or backpack.
You can find all kinds of logos/decals online in a variety of colors and graphics. Just google for 'reflective helmet decals' or 'reflective stickers and I'm sure you can find something that she'll love.
I just ordered a roll of white, pinstripe reflective tape from eBay so I can create a pattern on the back of my white helmet. You can find tape in all different reflective colors and widths if you just google.