Dear :

Please accept my apologies for the way you were treated [time] at our [store, office, etc.]. [company] will never tolerate any rudeness to customers, and the person who was involved in this incident is now no longer employed by us.

Thank you for bringing this matter to our attention. I sincerely hope this incident does not cause any damage to our business relationship.

Sincerely yours,

[name]

[title]