Oh my! First of all, communicate with the client that since this is his mistake he may be in for an additional charge. If you do the cake again, that is a cake you can’t do for someone else and you deserved to be paid for that. That’s the business answer.
Now technically, you might be able to save the cake. I would start by loosely placing the cake in a plastic garbage bag and tie the top closed with a pipe cleaner. Make sure the bag is loose all around the cake. This bagging will help minimize the condensation that’s going to occur. Place the bagged cake in the freezer and think positive thoughts.
About 3 days before you need the cake, move the bagged cake from the freezer to a fridge / cooler. Next day, remove from fridge and remove bag. Place at room temperature. You want to slow the thawing process down as much as possible. And once it comes out of the bag, don’t touch the cake until the fondant completely dries, which is going to be a day or 2.
I keep fondant cakes in the fridge every week, but the freezing process scares me a little, especially with the fondant. If you decide to do this, I would like to hear how it worked out for you. Good Luck!!