I agree with all of you (that it is not appropriate to wear white to a wedding unless you are the bride), but I have to ask if this particular bride would care much? I mean, in this day and age, it seems there are more and more women choosing to have untraditional weddings, in whatever sense of the term that may be. I have a friend who wants to serve Qdoba at her wedding, and have doughnuts for dessert. And she has no interest in a “traditional” wedding dress. I highly doubt my friend would care if someone wore a white sun dress to her wedding.
However, this is only given that this wedding takes place in the summer, or on some beautiful tropic isle. If this wedding is taking place in any other season, I would find it completely inappropriate to wear a white dress, regardless of the bride’s feelings.