That's hard. I would get the three of you together and apologize for upsetting them (but not for anything specific you did or did not do/tell, etc.). Maybe next time you can ask someone their preferences as to whether they want to hear what is being said about them. Not everyone does and it's a good idea to ask people about themselves instead of assuming they are "like" us and have our preferences. And/or you can go to the person saying bad things and suggest they say it to the person's face, let them know you don't think it is a good idea that they are talking behind the person's back.

I imagine some of this might blow over in time but the teller might make sure in the future that you're not around when the telling is going on and let others know you report what is said behind people's backs. But I think the person being talked about will learn that the teller can't be trusted to be kind, if not immediately, also over time. Leopards don't change their spots :-)