A colleague asked how to deal with this situation. “It's clear Brad is being a jerk. I'm working with him on how to be less of a jerk. But Susie asked me today when I'm going to do something about the problem–nothing she says seems to make a dent in his behavior. What can I tell Susie?”
Nothing. Not anything at all. You can say, “I'm working on the problem.” That's it. And if you have to fire Brad, you say something like, “Brad has decided to pursue career opportunities elsewhere.” If anyone asks about Brad's jerkiness problems, you say, “Brad has decided to pursue career opportunities elsewhere.” (That's the management equivalent of a “no comment” answer.
You can't say anything because even if you comment, how will they know what you say about them? You create a lack of trust by commenting about other people.
So, never ever talk about other people's performance.
This is #11 and #12 in Memo for Bosses: 101 Ways to Prevent your Office from Hating You.