Wow... the more we get into this... the more complicated it gets. Here again, I have to emphasize I'm not a mental health professional. So anything I say is just my personal opinion.

However, with this new information, yes I guess I would have some concerns with regard to what is occurring in the forum in question. I can't even give you a specific reason or rationale for my concern. But the fact that you have this one member who has now emerged as something of a "leader" begins to make this take on the trappings of an on-line cult it seems to me.

Perhaps this is just all fun & games for the members who are participating. One would like to think so. But, heaven forfend, what if the "leader" suddenly began issuing orders to the followers to do something that could cause them harm? And what if even one or two would feel the need to comply? Call me paranoid. But I'd have to wonder what your website's legal exposure might be in a situation such as that. And, of course, you have to have concern for those followers who might come to harm in the process.

All of this may or may not be harmful to the members in question in terms of their mental health. That's something neither you nor we can know for certain. But, at least from my perspective, this sounds like the kind of situation no good can come from. No ill may come from it either. But the potential is there, it seems to me. So, that said, perhaps it would be best to simply close that particular forum down & allow the members who post there to continue to post in other areas of your website as long as they abide by the website's rules. Then "punish" any who offend in the same way you would any other member.

I personally doubt any of us, here on PC, can really say definitively, what effect continuing to allow this, or stopping it, may have on the members who use this particular part of your website. But the more we get into this, the more red flags come up for me.