K.W.
One of my businesses is a professional event planning company, so I have a little experience with this- I would NOT do it myself but take it to a professional dry cleaners to steam and stuff the entire dress so that by the time she is ready to put it on just before she goes down the aisle, it's ready to go. You can wait until the week of to do it or do it now - the dry cleaners that do this on a regualr basis have techniques on how to pin the train so it does not re-wrinkle.
Good Luck!