She could always go as Miranda Keyes...except she dies in Halo 3. Prolly shouldnt tell her that part. All you need for that is a naval uniform with UNSC badges, rank, and some war medals. I think that would be pretty cool. Although Lara Croft would be cool, or Fox (Angelina Jolie) from Wanted. Or maybe Selene from the Underworld movies, she was pretty insane, and all you would need for that is a "leather" suit and a couple replica pistols. Maybe Scarlet or the Baroness from GI Joe...and I think that officially exhausts all my ideas lol. Hopefully you can find a good one for her to use.
EDIT: Dang, Nintendude beat to suggesting Miranda lol