A cake is only freshest for 3 days.
Do NOT put it in the frig. that will only dry it out more.
It is best if you put it in a cake box then wrap that in a plastic bag OR cling film & foil to freeze. Take it out of the fzr the night before pickup (or just several hrs before p/u) leaving it wrapped up. By the time it's p/u it should be defrosted and the customer will have a fresh cake.
But do be aware if the fondant doesn't have air circulation when not in the freezer, it will get very soft and limp and sometimes very sticky. I'd let your friend know the possibilities just in case, so she has no surprises.