Once you cover those cakes in buttercream and especially fondant, they will definitely stay fresh. How to fix it? You will get some (bad) customers that are all about getting as much money back as possible, but I know that’s not the case here. In this case, I would call the client and offer a heartfelt apology and tell them WHY this won’t happen again. Maybe something along these line:
I just wanted to apologize about the photo shoot event on xx/xx/xx. In this case, I over extended myself and had some time management issues, and the result was that you didn’t get my best work. I know you were disappointed; I was disappointed with myself. I am still new to this business and I have revamped my scheduling and prep schedules so that this type of situation won’t ever happen again. I understand if you are hesitant to trust me with your business again, but I would love another chance to earn your business.
It is very hard for a fledgling cake business to learn all these things on the fly while still making a good name for yourself. Keep you chin up. learn from your mistakes, and keep making cakes!
PS – One thing I did think of later… if this was for a photo shoot, did you ever consider doing a dummy cake? You could have worked on it weeks in advance and then kept it to showcase your work afterwards.