D.S.
I would tell her that I bought "special outfits" in advance before she buys hers to try and beat her to the punch. If she still bought one, tell her that you are going to dress her in the one you bought, but that you will take a picture of her in the outfit she bought so that she can still see it on her. Maybe you can make a scrapbook of the baby in all of the outfits she buys so she has something to remember them by. Good luck.