Last night I slaved away on a huge, fancy, princess first Birthday cake complete with ruffles and tiara. Today my customer's pick up time came and went, and when she finally got ahold of me, she realized she forgot to tell me that they changed the date of the party to next weekend!!!! She is still picking up the cake, but we want to know what the chances are that the cake will be any good. It is under buttercream and fondant- completely sealed. I suggested still putting the cake out, but having secret sheet cakes on standby in the kitchen if the cake ends up being bad. 

SO HAS ANYONE EVER- FOR ANY REASON- EATEN WEEK OLD CAKE THAT HAD BEEN SEALED UNDER FONDANT? If so- how was it?

Some do this as a matter of practice with wedding cakes, but I wouldn't recommend it. I have eaten some deathly dry cakes. Some have reported successfully wrapping the entire box and freezing it giving one day in the fridge to defrost and then a few hours on the counter and not unwrapping until time to put out and serve. They say the cake tastes as fresh as just made. Have not experienced freezing a fondant cake personally, but many say it can be done.

I would have her freeze it. Wrap the box in plastic wrap, and a couple layers of foil. It needs to defrost slowly, so once pulled from the freezer, it should go in the fridge, and the day of the party as early as possible to the counter. I have frozen and defrosted cake with no issues and they tasted great. The foil and plastic wrap need to stay on the box while it is on the counter so that the condensation forms on the outside of the box and not the cake.

What pastrylady said.  Freeze it well wrapped.  Yes, it will be good if you follow that method.  Do not keep it on the counter.  Keeping it in the frig isn't too bad but could dry out depending on what recipe you used (even well wrappped in plastic.).