Realize that you can't convince someone to leave in the course of a conversation, but that you can make the little crack that opens up years later.

The loss of the guy at the center of a cult of personality can quickly disperse the cult. Very rarely, they're able to transition into a cult of an idea rather than a personality. That's a difficult transition to pull off, and there's usually planning ahead of time.

There's not a lot of quick solutions otherwise.