Re:relatives Beren: I agree with the others. You've stated your case, and that's all you can do. Interfering further is just going to make both your relative and your friend turn against you.
I'm thinking that if you can't beat 'em, join 'em. Every now and then, there are situations like these where the guy actually does end up turning around and cleaning up his act. My stbx-father-in-law was an alcoholic, but somewhere early on in his marriage, he got help and turned it around. They've been married for over thirty years now. I guess the irony is that although his wife stuck with him and is now very happy with him, his daughter, not enduring 1/100th of what her mother dealt with, couldn't see staying with me. She certainly could've done worse, and come to think of it, probably will.
Wish your relative and your friend well, and do whatever you can to support their union. God knows, it sounds like they're going to need all the help they can get.
Beren