Most people join cults for community and structure and answers. There’s resources like psychologytoday.com/…/the-definitive-guide-helpin… and www.peopleleavecults.com/post/help-cult-involved but I think the general idea is stay calm don’t crash out when talking about it, help them notice the manipulation techniques being used that are common to cults, asking questions that help undermine indoctrination and inspire more skepticism, and provide alternatives for the reasons they are wanting to join or stay in a cult without the manipulation and lies a cult requires to exist.