At an adult webmaster convention?? don't you think that is a terrible place for an intervention??
Someplace away from the party is how you're supposed to do it, and they have to want it anyhow.
If they are going to an adult webmaster convention, drugs are on the schedule already. I would leave them to their own biz.
Interventions never work. Probably some of the best advice in this thread from seasoned users.
