My husband got me the time turner necklace for my birthday last year. I never wear it, it's more of a collectible, but it's beautiful and it represents the series I love so much. :) If she has a house preference - like considering herself a Gryffindor or Ravenclaw at heart - a sweater or shirt proclaiming that house would also be nice since it's winter.
There's a lot of Harry Potter apparal you could get her. I know the Hot Topic in my town sells everything from shirts, to bracelets, to bookmarks. But you could get almost anything online as well. They sell more options for candy online, if you're interested in getting her anything mentioned in the series. If you have a Candyopolis in your area, I know they sell Harry Potter candy as well. Ours only sells Bertie Botts Every Flavour Bean.
Generally, Harry Potter fans like all things Harry Potter. :) You could attempt to get her a wand, even. I'm not sure if you could customize it, per se, but I'm sure there's got to be a site on the web that allows that.