As someone detached from the situation it reads absolutely fine to me. It's not preachy at all and sounds very caring. As you know know they need to make the decision to quit for themselves and I remember being quite defensive when people talked about alcoholics, not even directed at me, but just in general. They may not respond positively.
I have a friend who I'm terribly worried about but I haven't even mentioned it. I'm just getting on with my life and leading by example. It sounds like your friend had an issue with a partner's drinking so perhaps this email will be the message they need to read to start the ball rolling.
I think your friend is lucky to have you in their life.
S x