Here’s a bit from an article published at The Conversation:

The most important piece of advice is to not criticise, condemn or judge, even if you have serious concerns. Instead, focus on why this person identifies with the group so much, and what they believe they are getting from it. And try to reinforce the message: “It’s great that you’re developing yourself and your skills so positively and that the group is making you so happy.”

It may feel cheesy, but the point of this approach is to draw on the psychological technique of motivational interviewing, so that these positive statements, similar to those the person has made themselves, will eventually lead them to question whether they are really true – we call this the “strategic and personal oriented dialogue” approach. This means you have to keep talking. Keep the dialogue going and help your loved one measure the group against their own hopes and standards. In time, the scales will start to fall from their eyes, and you can be ready for that moment. [continue]

The authors are Rod Dubrow-Marshall (Professor of Social Psychology and Visiting Fellow, Criminal Justice Hub, University of Salford) and Linda Dubrow-Marshall (Lecturer in Applied Psychology, University of Salford).