I would probably get fired from your bakery, because it'd be tempting to tell Cakeasaurus "The cake was the wrong flavor ? So you ate the whole thing anyway ? And you think, what, someone hacked your email so they could see you eat your least favorite flavor ? Tell me, how many slices did you personally have ?"
No you wouldn't, because this is pretty much the conversation that we're going to have if she comes in One of the reasons we insist on having the cake back is that we want to SEE the problem and SEE their face when they try to lie to us. Here's the thing, the decorators are here when people pick up. If there is a problem with a cake we can fix it immediately.
She may think that if she pitches enough of a fit, you'll refund her $$$$ or a good chunk of it. She got to impress the party guests with her display of extravagance, and now she doesn't want to pay for it. Because a couple of dozen cupcakes baked by her own little hands, or a $15 cake from the grocery store bakery, doesn't have the flair that this special cake did. She may think that you'll cave because a lawsuit would cost you more than refunding the cake.
If she refuses the charges on her credit card, I'd put that shoe on the other foot, and take her to small claims court.