Wow. I think this is why there is a minimum number of servings set for tastings. I only do tastings for wedding cakes. Never had requests for tastings for small celebration cakes. You could always charge them for some cupcakes in the flavour they want to try or give them two complimentary cupcakes. Depends on how your business is designed. Complimentary or "we charge".
I'm not surprised you didn't hear from her. "The customer is king" is probably what they thought about getting to taste your cake. Would you be able to offer cupcakes next time you get a request like this whether complimentary or at a fee? This is going to keep cropping up until you have a well-established brand. You don't want to lose out on business because of this. You could freeze a batch of baked off cupcakes and pull 2 from them.
We provide tastings for any size cake. The tasting consists of a to-go package with 2 unfrosted 6" rounds and 2 small containers of frosting for $30. Percentage-wise it is one of our most profitable products.