If you meet someone who is clearly know what to love and what to hate, and he asks you to stand on the same side with him, you can just let him enjoy venting his dissatisfaction. But you have to control yourself that do not say something to make it worse. You no need to fawn on him to slander others. If you say something you should not say, maybe he will tell tales in the future.
What you need to is try to ease his emotions, so that he will more likely take you as his friend. He just need you have the same feeling of him.
It is like when your friend talks to you that he lose money in poker gambling game, you should not tell him that maybe other players are using the Infrared contact lenses, and with the invisible ink marked cards that they can easily win in gambling. Or the normal cards reader, perspective poker table and so on.