I think you should play it safe, and buy her another bottle of no. 5. I don`t know if she wears the Edt, EdP or perfume, but if you can afford it, I`d recommend perfume.
If you buy EdT, maybe you could give her the matching body lotion? If she "always" have worn no. 5, and nothing else, I don`t think this is the right time for big surprises.