She LOVED the cake, but she had to postpone the party from Saturday to Wednesday of the following week. She picked it up on Friday and I baked on Thursday, so by the time the cake is eaten, it is 6 days old. It is a white cake filled and frosted with chocolate buttercream and topped with marshmallow fondant.

I suggested that she put it in the fridge until then, and 1 hour before the party take it out. I also suggested leaving the cake out during the whole party so that any condensation can dry in open air. Did I tell her the right thing? Do you think the cake will be okay?

A similar thing happened to me in June. I made a giant cake for my friend, and his whole family was sick so they had to push the party out to the next weekend. I told him to freeze it in the box, and wrap the box well with plastic wrap and foil. Then thaw it out on the counter but leave the wrapper ON. That way, condensation would develop on the wrapper and not the cake. He did just that and told me it tasted freshly baked. Freezing for the win!

A friend of mine once picked up a cake wednesday morning for a sunday party. It was chocolate cake with ganache covered in MMF. She told me that she left it out in a cool area for the entire time and it was divine, like it had been baked that morning.
The ganache and fondant help keep the cake moist and stay fresh for longer.

Be sure to emphize not to touch the cake at any time if there is any condensation!
As long as you baked a nice moist cake it should be just fine if they fz it.