Popular in the US for some years, and increasingly offered in the UK, an intervention is a sort of 'surprise party' for an "addict", where their closest friends and family gather with (usually) a couple of professional interventionists, and they are strongly encouraged to enter treatment, usually residential rehab.
Imagine all the characters in Winnie The Pooh meeting up with Pooh and saying 'Pooh, we're all here today because we love you, and we're very worried about your destructive relationship with honey'1.
The language is kept respectful, direct and non-confrontational.
My concern is that the people providing the 'intervention' are in effect sales reps, as they probably either work for a rehab, or take a commission from the rehab. I see advertisements in the trade press from intervention specialists, who offer 'increased admissions' and I don't think they do this for free.
See also: Referral agents
1 I nicked this from a very good John Finnemore sketch on BBC Radio 4