You would only be required to refund her for the unused portions if your terms of service or sale--i.e. the terms in place when you bought these sessions--provided for a right of refund. Refunds are not mandated under the law (at least as long as you providing the service which was purchased), even if the client or customer is dissatisfied; they are only required if the business has adopted a refund policy.That's the legal answer. Speaking as a small business person as a well as an attorney, consider whether the potential bad word of mouth from a dissatisfied customer who would feel you are taking advantage of her is worth whatever amount of money you'd gain by keeping the balance of her payment.