You're having dinner with people you respect and admire. They all criticise a close friend of yours (not knowing he/she is a friend). The criticism is unjustified. What would you do.

Depends on the nature of the criticism/gossip. How I might gently inquire as to why they came to these conclusions and/or refute them would entirely hinge on what they were focusing on and how much knowledge of that i had regarding said friend. Odds are, I would not let it lie, but neither am I going to tip the table and storm out in some fit of righteousness.

Have at it.