I love the initial question and I have been thinking about it over the last 24 hours.
Knowing what to say could help so many people going down the same track.
It is a really great question. The problem is usually when people start drinking and drugging, it is happy using and can take a while to get to that ambivalent with causing a few problems stage and a while longer to the unhappy user or drinker.
I don't think we realise how much power we have even at the unhappy stage of addiction and we don't tend to believe a lot of those who tell us we can do it.....and I am still thinking about what I could say to my pre-addicted self that may have helped.