We left port last September 14th for my daughter's wedding at
Castaway Cay. This was during Hurricane Isabel.
I can't speak for what may come with Hurricane Ivan....but as far as Frances....by the time the Magic gets to Castaway Cay (I'm guessing around the 16th of Sept, Castaway Cay will be back to normal...minus a few trees, maybe.
There is no other beach for Disney to do their weddings on. The weddings at Castaway Cay aren't "on" the beach, anyway. They are held at the Head's Up Bar (which is shuttered closed during the ceremony, and you'd never know it was a bar) which has a large dock, all with railings around it. The ceremony is out on the dock, with the ship in the background.
If the ship doesn't dock at CC, the wedding would most likely be held at Palo or Sessions.
Either way,
DCL's Fairytale Wedding co-ordinator should be contacting the bride very soon to update her. If not, then the bride should contact the coordinator.
When we were dodging Isabel, the ship went to Castaway Cay the first day of the cruise, instead of the last. We were called about 4 days prior to sailing to let us know the change of plans. It turned out to be a good thing, as we got the stress of the wedding over and done with, and could relax and have fun the rest of the cruise. But this is why DCL states in every piece of info they have about the weddings at sea, to not have a date pre-engraved on any items, as the date could change.
As far as we were concerned, Disney did just as beautiful job with the wedding as anyone possibly could have, anywhere. It was absolutely beautiful, and they took care of every detail. My daughter wouldn't change anything about it, and neither would we.
Your bride friend will have a marvelous wedding!