Don't try to challenge their belief system. The best thing to do is to over-validate their position but without any trace of sarcasm. For example, "Your boss should be fired. It's terrible that there's absolutely nothing you can do to make things better."

Once they feel understood, there's not a lot more to say. They will also hear their own complaint from you and may turn the argument. "My boss is awful, but I don't know if I'll be there forever."

They often suffer from an underlying depression, and depression skews their thinking and make them feel helpless. They also feel lonely, unheard, and unseen in their pain.

They want to connect, but because they are help-rejecting complainers, they push people away, creating a vicious cycle...