grannywonder Agh! I am in agonies for you! This is truly weird and definitely not very thoughtful of Bride-to be! Still, it takes all sorts...
Here's me, trying to be a Christian, seriously thinking: Is this a time for a little white lie? Can you draw this out as if you are doing as asked.... until at the last minute, the dress has to go back with a big problem like the back seam is coming undone? I'm NOT saying actually buy it and damage it! That is too dishonest! But is there some kind of cover story you can think of, that can make it seem as if you were - sadly(!) - unable to wear the dress on the day?
This is all I can think of right now. Just saying, "Sorry, I tried it and it simply isn't me, I don't have the confidence to carry it off" should be ok for most people, but probably won't work on this Bride to be and her mother, I suspect.
Good luck