Of course mistakes happen but there is a way for the business to handle it so that the customer doesn’t get pissed off and doesn’t want to come back again.
If this is a loyal and big spending customer then a $10 discount is laughable and insulting. The manager should have thought long term and said ‘your next session is free’ and I bet the customer would have accepted and it would have stopped there. It amazes me how short sighted shops are and try to penny pinch and end up losing more business. Big spending customers are not easy to find so a smart manager will try to keep them happy and sometimes even give some back to customer to maintain good relationship.