Don't get sucked into playing mom or dad. The goal is to develop an adult communication environment. Explain to the person that if you get involved, it is likely to make the situation worse because the other employee will be angry about the supervisor's involvement. Help the employee with coaching and advice. Frequently, the employee has come to you because he or she doesn't know the words to say.
Role play if necessary.
Then schedule a follow-up meeting to hear how the conversation went. The employee will feel that you are helping him instead of pushing him away. It also holds the complainer accountable for following through with the plan to meet with the co-worker to talk it out. Use firsthand examples when possible. If the situation worsens and you find that you must get involved, it's important to use examples of firsthand observations you've made about his or her behavior.