Here's a question:
If you overheard your colleague Mary saying something negative about your friend and colleague John, should you tell John? Or if you hear from someone that Mary has been saying negative things about John, should you let him know?
My answer is no. Always, no.
I've been in a situation like this before, where I was tempted to say something, but I always decided that it was none of my damn business. Partially because I've been Mary before, and things I've said were misrepresented and taken completely out of context (I don't want to get into that story at the moment). But this is my favorite example of why you shouldn't do that:
In residency, there was an attending at my program named Dr. Lee. I worked with Dr. Lee and I liked her. I thought it was mutual.
About a month after I came back from maternity leave, I was informed by a co-resident that some other residents had been complaining about a certain female resident (not me) who had been calling in sick a lot on Fridays and being irresponsible. Dr. Lee quickly chimed in, "I know who you're talking about! It's Fizzy, isn't it?"
I was so upset over the fact that Dr. Lee would think the worst of me, and assume I was the irresponsible resident when I never missed a day when she and I worked together. For a while, I was very angry at Dr. Lee.
Several months later, I talked to one of the residents who was involved in that conversation. He informed me that:
1) The attending involved in the conversation was not Dr. Lee at all, but was Dr. Green.
2) Dr. Green did think they were talking about me, but chimed in to *defend* me because I'd just had a baby and thought they should cut me some slack.
So basically, the story was wrong AND the person involved was wrong. I wasted way too much energy being pissed off over that. And all because a well-meaning person felt like he should give me a "heads up."