I would respond by first apologizing for the way they feel they have been mistreated/ mislead/ unhappy by the company. I would offer to make things right & reach out to discuss further as to what occurred and what we can implement as a company to make sure it doesn’t happen again. I would assure the customer the negative they experience they had should never happen again and give them a discount / replace product for them to try out the company another time & have a positive experience!