If you let the verbal trash from a co-worker pile up, it will not take long for your job to have a rotten atmosphere.
As a result, your first step should be to meet with this individual. However, remember that all you know is hearsay from your friend. This means that when you meet, do not flip right into the attack mode. Rather, tell him that you are concerned about the working relationship and would like to know what he thinks would improve it.
His response will let you know what you need to do next. There may be an explanation, denial, accusation, some questions, or even an apology. Perhaps the matter will end here, or with a commitment from both of you to work more productively together.
But, if you get nowhere with this employee, then you should try to get somewhere with your manager. If your manager is still hesitant, perhaps he needs to be shown how he and the department will benefit if this trashy matter is cleaned up.
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