Assuming this story is true, she needs to put her hand up as soon as possible (jpcher is right that everyone else will reply 'It wasn't me!', and being found out always makes a person look like the weasel, whatever the rights and wrongs of it) and respond openly on Facebook saying 'I saw your message about the 5-day trip to Houston* and realised that being a bridesmaid is going to cost way more than I expected or can afford. So I called the store as a first priority in the hope that the dress could still be cancelled. They told me no, but even if I'll be out the cost of the dress I'm still going to have to step down from the wedding party; I just don't have that kind of money. If you find a replacement bridesmaid and she's near my size I'd be happy to sell the dress on to her at a discount'.
Then let the chips fall where they may. Just possibly the bride may realise for herself what a bridezilla she has been and rein the whole thing back. Possibly the other bridesmaids may be encouraged to pipe up saying 'yeah, we can't really afford that kind of jaunt either' and force that realisation on her. But even if the others don't back her and the bride is bridezilla enough to unfriend her, disinvite her from the wedding, etc, then that isn't a friendship worth keeping anyway. Better lose it now at only the cost of a dress, than go through with the whole thing and end up broke, exhausted and bitterly resentful of this woman she no longer likes.
* IMO she shouldn't mention the bridal registry thing and the demand for individual presents at all. One of the injustices of life is that there's no way of saying 'I wouldn't dream of giving you anything as expensive as that' without looking stingy, no matter how unreasonable and gimme-pig 'that' was.