well if you offered to change your dress and she was still saying she would return hers, I also think that maybe she is having second thoughts on whether the dress was right for her in the first place...or, she doesn't know how to deal with the situation and doesn't want to come across as a bridezilla! You know your friend best, would she normally just come right out with it?
Being honest, I don't mind people doing alot the same as me, but if one of my friends chose either the same location, or same dress as me, i'd have a problem with it...but thats me personally. But again, if i didn't say anything and just kept it to myself, then it would be my own fault that i didn't address the situation.
xxx