Defend the person who isn’t in the room. One day it’ll be you who’s not in the room.

What happens when someone is absent is the real test, since there is no social cost to joining in and no reward for stopping it. Standing up for a person who will never find out is one of the few genuinely unwitnessed decent acts available. It also changes what the room treats as acceptable, right away. Everyone present quietly updates the rules.

When the conversation turns on someone who is not there, say something plain: that has not been my experience of them. No speech is required. One sentence usually ends it.