Right now she feels happy, and even a little loved, because she found a dress on sale. She probably feels good in it too, and because she thinks it's from God she doesn't have to feel buyer's remorse.

But you could tell her she just happened upon the dress by chance, and probable odds worked in her favor. You could tell her that it's ridiculous that God would get her a dress on sale instead of saving millions of starving children. But that would take away her happiness, and she would feel guilty about the purchase that otherwise hurt no one.

I would just leave it alone. Just because it represents everything wrong with the idea of God, doesn't mean you should make your friend unhappy. She's wrong, but she's allowed to be. Just roll your eyes and move on. You can't fix everybody.