I'm on a team of 10 or so people. About half of us are remote in parts of California, and half are in the office in the Bay Area. A few years ago, a specific team member, we'll say "Team Member 1" (TM1 for short), was overheard by someone in another part of the company telling people in our department that a few specific people on his team (TM2, TM3) were just not smart, didn't know what we were doing, and generally should just leave because he could do so much more if we weren't around. Well, the overhearer came back to me and said, "I was talking by your area today and I heard your name and <...> from TM1."
Because this is not my first job, and I'm in a more senior role than TM1, I immediately went back to our manager and told him what happened. I pointed out that we can't be undermining ourselves by talking trash about our own teammates. Our manager supposedly had a private talk with TM1, and it wasn't going to happen again. We eventually lost both TM2 and TM3 to competitors, and I remain.
Since this all happened, I've been promoted to a leadership role. Just this week, someone in my department mentioned that TM1 was doing it again. Not only that, but specifically was saying I wasn't pulling my weight and my leadership role was taking up too much of my time. The overhearer also told me it sounded very much like the previous things TM1 used to say about TM2 and TM3 before (I didn't realize there was a pattern before too).
Because this information comes from a gossip-y source ("someone overheard you"), I feel like it lacks credibility. I've been asked not to expose the overhearer in each case, as they didn't want to get involved (actually, the first OH almost went to HR, but the second OH person told me in confidence). As a leader, I don't feel like getting involved in the petty situation -- but I know this will continue and could have actual business impact if we don't cut it out (we could lose more people). But I can't really disclose my sources either. After this happened, I did re-confirm that my boss doesn't see any performance problem with my work, so I know I'm covered in that regard.
Do I betray the confidence of my friend and tell my boss what happened? I've already asked my friend to report it directly to my boss, or not tell me in the future, as it puts me in a terrible position (and I can't force him to tell the boss). Do I just find another team? Our business has a reputation of not having the kind of culture I describe in this story, and that's why I like working there. I'd rather not leave, but any attempt to resolve this situation potentially undermines me professionally and could violate personal trust towards me as well. I'm also concerned that my boss will handle it quietly and it will pop up again in 6 months (we will fix the individual behavior, but the culture will remain).
Posting anonymously, and will do my best to answer any follow up questions. If anything is specifically relevant to workplace laws in California, that would be valuable too. Thank you!