What can you do for someone who is a truly a lost cause?

I have a friend who, while not actually admitting it, I think has joined a cult. He is so rooted in his ideas, like the Illuminati and the Freemasons, Creationism, aliens actually being demons... And that is just the start of it. I know he smokes weed, but I think he may be into harder drugs as well. He isn't violent at all, as far as I can tell at least, but he refuses to even entertain the notion of being wrong.

I hadn't seen this friend in a long time, and if I forget about it now I will probably never see him again, but it shook me to see him like this. What should I do? I am honestly scared for his safety.