A question of Ethics regarding patient of dental office?

A Patient comes to a dental office and gets $3K work done. Insurance doesn't cover $600. Due to the good nature and as a courtesy the office writes off the amount not covered without discussing it nor a request from the patient. One year later the patient calls to schedule an appointment for her husband. Through a gross over- reaction on the part of the patient her husband calls and while using foul language tells the receptionist that he will not be comming back to the office. Several messages later the patient finally speaks to the owner of the business and though no foul language was used the attitude was rude and disrespectful. Even though it's been one year and the office deserves the money that was origially written off, should they bill the patient or just forget about it? The patient was made aware of the amounts not covered by the explanation of benefits the dental insurance sends out but was never billed and wasn't even aware of this courtesy ajustment or write-off until one year after the fact.
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So, you logic is something like this: you are a great patient and we love you and give you $600 discount. Now, a year later, we do not like you for whatever reason and we want you to pay us $600... Try it and you'll loose many more patients because this woman and her husband will tell this story to every patient of yours.
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