I know Ana- the point I was making, is that I know few people who have a small glass;)
Thinking about the OP some more. What about the aspect of 'guilt', as a good fried or relative. If you have a friend or relative whose behaviour, be it over drinking or eating (or under/poorly), or drink-driving, taking v. dangerous risks, smoking heavily, etc, etc- whatever- and you don't make any attempt at discussing this with them- when and if disaster strucks, be it via accident or illness, etc- then will you not forever be thinking 'if only I had talked to them about it- told them how their behaviour worried me- because I love them, because their family who love them asked me to try and intercede?
Thinking again about my dad, didn't smoke, ate like a sparrow, did tons of exercise, didn't drink- but continually took stupid risks clombing on rocks, going for massive walks or xcountry ski hikes in the woods, without a phone or telling anyone where he went, carrying huge pans of boiling water across the kitchen to save tuppence, climb on the roof to clear snow, driving too fast and erratically at 90+- etc- had something happened related to the above, and had I, we as a family, and with the suppport of friends and neighbours, not had done our best to try and curb these excesses- wouldn't we have felt dreadful that we didn't at least try? Clearly our own experiences influence the way we react in other cases. If you've suffered from seeing your parents being alcoholics- you will be extra sensitive to seeing others with the same problem. For me, it was the constant un-necessary risk taking with dad. For others it will be food- if their parents died of diabetes or other food related illnesses, etc, etc.
As it happened, my dad died aged 96 due to severe diahrroea (sp???) after being given unsuitable antibiotics as a precaution (!) in the OAP home where he was from age 95, as a few in-mates had the flu (he didn't!). He was so thin, just skin and bones, and had no reserves at all- he was gone is 2 days- heart attack as he sat on the toilet, and all alone, poor man.
But if you have tried, and tried your best, even at the risk of losing that friendship... then you'll just have to let it go. If you can't bear to watch, you will have to put some distance between you and them.