I baked cakes on Thursday, decorated on Friday for a party today that was canceled due to weather. Will the cake still be good on this Thursday for the rescheduled party?

Will it fit in your freezer?

Don't think so - it's already delivered to the client. I put a lot of fondant pieces on it as well, and have heard that wouldn't be good for the fondant.

Fondant freezes fine, even on the cake. I would suggest to her that she wrap the cake box in plastic wrap and put it in the freezer. Take it out the morning of the party and let it thaw.

Aren't you glad it's already delivered and you don't have to try to store it, or have the customer asking you to remake it at no charge for the party?

You can have the client freeze the cake.
It will be fine.
It is in her possession and you did what you were paid for so the cancellation is her problem.
Educating her on how to deal with it is just good customer service. As stated above, thaw day of event.
take care

And have her keep it wrapped while it's thawing so any condensation will form on the wrapping, not the cake.