The price of having an open and communicative relationship with your employees is that many are going to bring in bunches of information. Sometimes you can receive a bunch of information that is critically important and would have been missed if not for your accessibility. At the same time, your accessibility can also open the door to a bunch of garbage.
Since it sounds like you received the latter in this matter, the first step is to meet with the message bearer and present him with the truth. Then ask him what he thinks. If his reaction is one of contrition, and he has not been racking up mileage as "frequent liar," then the discussion should focus on his being more careful about what he says about other people. It will also be important to repair any damage that may have been done to the reputation of the individual whom he inaccurately described.
However, if the message bearer responds with denial or defensiveness, or if his comments are part of a pattern of maligning other employees, then you should formally document the matter and advise him of the serious consequences if there are similar incidents in the future. He needs to understand that you are open and receptive to his stories, provided they fall under the heading of important non-fiction.
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