The venue is asking for the sun, the moon and the stars. None of which is Will's responsibility.
Agreed, this credit card/deposit racket is completely out of line. I'd suggest taking this up with the client and see what they have to say about it. I understand it's short notice and probably too late to find another venue.
That said, the venue isn't likely to change their policy just because Will (or we) think they should. And the house could very well deny Will access if he isn't playing along.
If it comes to that, play along and watch your credit account closely afterwards. If the venue assesses a charge, dispute it with your CC company or bank. I am quite confident that, given the unclear circumstances the venue uses to threaten charges you will prevail.