You can listen to him/her complaints until you are certain that they feel heard out and listened to. Ask questions. Clarify their statements. Make sure that you have actively listened. Decide if you believe the employee or coworker has legitimate reasons for their negativity. If you decide affirmatively, ask if they’d like your help to solve the problem. If they ask for help, provide advice or ideas for how the coworker can address the reason for their negativity.