time and again this woman has been an extremely toxic influence upon his life. This will be the sixth time they've re-united, and each time they do he becomes more unstable - mentally, emotionally, and even physically.
Does he agree with either assertion - that each time she's been toxic, and that his life gets progressively worse each time?
You really need to question
* Your own motives
* Your experience and understanding of complex relationship dynamics
* Your perspective
How close are you? What happens to your relationship with this guy if he goes back to his ex? Do you like or dislike his ex (excluding the effects she has on him - is she a regular human being, just trying to get by, or is she the next hitler)? Are your own emotional needs fulfilled in a way that if he becomes unavailable as a friend you will be ok? If you're female, she may see you as a rival, even if you're merely a good friend. You'll have to respect her boundaries if he ultimately chooses to stay with her - and that may mean losing him. Hopefully this isn't the reason you believe she's toxic - because then she's only toxic to your relationship, and not necessarily to him.
Are you sure you have the correct perspective on their relationship? It's sometimes amusing to observe couples who've been together for decades trading barbs. One might get the impression that they can't stand each other, yet this is a practiced form of bonding and communication between them. Is the relationship itself toxic, or are you merely observing it through your own experiences and misunderstanding the relationship because it doesn't make sense to you?
What is he missing in his life that his ex fulfills? Does he have low self esteem (ie, is he unable to provide emotional support to himself when he is unable to find it elsewhere)?
I have no experience with "interventions" but I suspect that it's better to build a person up over time and increase their self esteem and their experience and knowledge than it is to have a big "You're screwing up, and we're here to straighten you out" party. They may be convinced of your correctness, and agree wholeheartedly with you at the time, and they may agree to any measures you take to "help" them stay on the path you've chosen for them. However you and many other people have to work hard for months and years to keep them out of trouble. An intervention seems to be less a commitment from the target to do better and more a commitment from all the participants to help the target do better.
But if you believe that's his only option (interventions seem to be the path of last resort) and that his choice will ultimately destroy him, then I advise doing research on what an intervention is, what it is meant to accomplish, what part everyone plays in it, how to set it up, get commitments from participants, plan, and run one.
It's not a small thing, and if you do it your own way with your current limited understanding of what it is, then it will
spectacularly backfire.
But, you know, that's just what I think, having no experience with interventions, and only doing some small amount of reading into them.
He is obviously already aware of your concerns (otherwise you would
never consider an intervention) and he's still choosing this path. Personally, I'd tell him something along the lines of, "Look. This is what's happened the last few times you've engaged in a relationship with her. I don't think it's good for you. However, I support
you and if this is the choice you will make, I wish you happiness in it. I'm not going to talk badly about her or this relationship again, and I'll support you as best as I can."
Then do that. You may not be able to stop it, but you can build his self esteem and make sure that no matter what badness happens in his relationship with her, your relationship is good.
