Don't say anything because first it's gossip which you're spreading for this person and second, its not anything you want to be party to because it makes you culpable. Whatever has been said, just let it go. Moving forward stop them before they start in on something else. Just remove yourself from that dynamic and separate yourself from the spotlight this person is trying to get you to share with them.

Also remember, people who are trash talking about others to you are likely trashing talking about you to them. People who instigate conflict, love to set up sides to control the narrative. The best way to avoid that is stay out of it completely.