If she likes Pokemon, she could always be a Pokemon gijinka of her choice.
There's always White Mage from the original Final Fantasy. (Recognizable, pretty easy, and anyone can wear it!) - http://unrealitymag.com/wp-content/u...bead_art_3.jpg
If she likes Disney, maybe the Fairy Godmother from Disney's Cinderella? (She looks nice in blue, so this would be really cute!) - http://images1.fanpop.com/images/pho...37-250-212.jpg
If she enjoys the Mario series, she could get a pair of blue overalls, appropriate colored shirt, and a hat to be either a female Mario or Luigi (whichever is her favorite). She'd just have to decorate the hat with the symbol.
Or alternatively, she could get white pants, a white lab coat, stethoscope and appropriate headgear and be Dr. Mario - http://www.mariowiki.com/images/thum...rioAdvance.png
If she likes any of the games the rest of the group is dressing up from, it might also be fun to try to come up with a costume to include her with someone else in the group. (Assuming she also likes those games, of course!
Hope these ideas helped a little!