Be a good friend, in small ways, to keep contact. What might that look like?
Be honestly curious about what he is experiencing, and ask questions from that place, not trying to "de-goof" him. Support HIS thought process-- don't angle for him to change it for yours-- so that he can think about things for himself.
Remember that it's his life and that he can handle it, and remain a loving and available support person even if he disappears for awhile-- be a good place to come home to when he's ready.
Be a good model of having a sense of humor about oneself. Cults are so SERIOUS. Be fun, off the topic of the concerns.
(And maybe at the same time, quietly investigate the situation and see if you have any information about illegal activities of the higher-ups, that you can pass along to the right people?)