Acknowledge the customer's complaint, apologize for the rudeness and indicate the corrective action to be taken. Stress that this is not normal behavior for your employees.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I would like to apologize for the rude treatment you experienced from one of our employees on <date of incident>. The behavior of the person involved in no way reflects the standards to which we hold our employees. We <corrective action taken>.
I appreciate you bringing this matter to my attention and hope that you will give us a chance to make it up to you. <incentive> Thank you for your understanding. We look forward to serving you in the future.
encl: <List of enclosed items goes here>