First off, don't buy her a helmet based on graphics/pretty colors. Take her to a dealership or a store like Cycle Gear that has a lot of helmets in stock, so she can be professionally fitted and see what brands and shapes fit her head properly. There are lots of great brands out there -- Shoei, Arai, Suomy, Shark and Scorpion all make great lids, and each has a few different styles and shapes. TRY before you buy.
Once she knows what brand(s) fit her the best, then have the store pull out the catalogues for those brands (or shop online). "Girly" helmets are becoming more and more popular... if she wants pink, purple or light blue, she'll have a million choices. And if she wants something more neutral, there's some beautiful elaborate graphics available from Shoei, Suomy and Shark.
(Personally I wear the Arai Haga "Fire & Ice" which is black, white and red -- it's not manly or girly -- it's just WILD and colorful).