What type of goods does she plan to buy? Do you have a general idea? If she's in the market for clothing, then I'm probably thinking about one of the big Euro fashion meccas. Milan perhaps? If she's into jewelry, then I don't know...is it still Rotterdam for jewelry?

If she has a variety of items on her list, then I don't see how you can go wrong with London (for the more alternative stuff, outdoor markets, etc.) or Paris (more fashion, etc.).

That said, I've never personally planned a trip around shopping. These days, you can pretty much acquire everything you want online anyway. So I'm pick a cool city and mix in an excursion to the best local markets in the area. But my wife keeps reminding me that I'm a guy and can't possibly understand women's fashion (let alone teen fashion!), so don't listen to me.