Try to reason with him without seeming condemning or preachy, get him to see that the cult might not be that good of an idea (if the cult is dangerous or something; i.e. violent, suicidal). If the cult is just some weird as cult where people get together to do weird things, I'd let them do it for the most part. I'm not going to try to interfere with what people believe, unless its teaching them some really fucked up things (rape, torture, kidnapping, murder, arson, theft, free-thinking {I kid, I kid}). Although if the cult seems to be a big enough danger I might report it to the authorities. Might save some lives.