If the box is really well wrapped and the freezer operates consistently, the cake will be fine for many months--at least 6.
Now, if the client knows that it's been frozen, I'd let her choose what to do with the cake. I'd get hold of her on Jan. 2 and I'd tell her that she has 3 choices--one to be made IMMEDIATELY---pay for the cake & take it NOW [to do with as she pleases]; pay for the cake and, if the wedding is held in the next 3 months, use the cake [as is, with no option for complaints or refunds]; pay for the cake and allow me to dispose of it as I see fit.
If the wedding is definitely more than 3 months out from the original date, I'd tell her that she has to pay for this cake NOW and that a new one will cost her the same amount in the future.
Of course, you could also have her pay for the cake and take it to her own freezer and let her decide at her leisure.