This whole fiasco seems to be misunderstanding for the most part.
The casting director of Louis Vuitton has provided their side of the story. Her clothes were custom made to her measurements, unfortunately when she arrived in Tokyo, the outfit didn't fit her any more. They didn't have an atelier in Japan, so they couldn't alter it on the spot, in the end they had to leave her off. Also, they never told her that she could only drink water, they told the models to drink lots of water because after a long flight, they might suffer from dehydration.
Their way of handling the cancellation was probably poorly done, but it was not like they didn't give a girl in size 6 a chance as they did give her a look to walk for the show.