This seems harsh! Why is the girl a manipulator just b/c she doesn't want to go to Disney? Who said the trip was always a Disney trip? They said they dreamed of a trip together since K. I would think that when spending a lot of money they wouldn't expect the girl to go somewhere she didn't want to. Is Disney really the end all be all for these girls? There is no place that the three girls would want to go together? I would say see if they can find something else, maybe even a
Disney cruise would be a better option. I don't think the girl is trying to manipulate anyone. If I get together with a few friends for dinner and suggest x restaurant and one says sure and the other says no thanks, I don't like that place we then try to come up with something we all like. I don't just drop someone who I have been close with for years b/c they couldn't agree with us. Everyone has a right to have input when it is there time/money and I think it is kind of selfish to just ditch this girl unless she is being difficult and won't agree to anything or is demanding one place or refusing to go. To me the trip is about being with friends and there are more places to go and have fun besides just Disney.
OP, I would leave the girls alone to work it out. I would ask my dd if there was a way to compromise and find another destination that would work for all of them though and if she would rather they all went somewhere together or if only two of them went to Disney.