When faced with a negative client or critic of any solution or service that the customer provide him how to behave as an employee ?

First of all before providing any service or solution to the customer, the employee should put himself/herself in the customer's shoe to better understand how the customer is feeling? How important is to solve his issue? And what is the best way to solve the issue while satisfying and delighting him. Now if you receive a negative feedback or criticism from the customer, you must show empathy to him/her by relifeing them, telling them you really understand how they feel about it and you will do your best to solve this issue in the best way possible. Also a good approach can be by asking the customer in a polite and professional way about the solution which will make him happy. If the employee is empowered enough to take decisions, then he will do it otherwise he will escalate to his superiors for approval then get back to the customer.

Always deal with the customers with positive view. Analyze and understand where the criticisms/thoughts are coming from (may be the customer experienced poor services or unstable solutions before) and then discuss the quality of the service/solution and how will it bring a good impact in the business / client-side.