Hi Jayne,
You can freeze the cake. I have done it before.
I would recommend freezing it in a cake box - the cardboard boxes, the white on the outside and the cardboard (gray) folded on the inside.
Do tell your customer that it will be frozen, that is important.
24 hours before needed by the customer, the cake can be taken out of the freezer and placed to thaw in the fridge. Do not open the box as you will probably be alarmed at the cake sweating. This will dry up and be ok, once served.
I am not sure what kind of decorations you have on the cake itself. Are there any 3d figures? Of there are, try and take these off beforehand. By 3d i mean like bridesdolls or cats or large flowers as these may not fair well in the freezer.
Good luck!
If you have more questions, please don't hesitate to ask.
Groetjes Tweety!