If she kept the cake in the car all day, then you didn't need to refund the money. But once you decided to refund the money, and told her that you were going to refund the money, then you need to do that. Because now that your husband stepped in and said she can't have the money, she's angry.
Sorry that this happened to you. Maybe you should verbally tell them to keep the cake refrigerated, even though you have a sticker on the cake that says "keep refrigerated".