Go ahead and go to the show *but* ask for a partial refund that reflects the snake's actual value.
[other dealers at shows will usually honestly identify/appraise your snake for you]
I seriously doubt the seller wants a crowd of onlookers and potential buyers overhearing their "mistake".
If they're honest, they'll cheerfully agree to a partial refund.
Either way, you win.
Your daughter gets to keep her snake, regardless of what the seller does and you may get a sizable portion of your money back.
It may not be a pleasant thing for you to endure but think of all the "newbies" who'll come after you laying down good money for an animal that isn't what it was supposed to be.
You could very well stop a potentially crooked seller in their tracks.
I certainly wish I'd taken an "expert" with me to my first show.