Addiction is not just a chemical dependency. The context in which you use the substance matters. Your personality and past behavior matters. This friend, in particular, took partying to an extreme. Not one drink but twelve drinks and blackout situations. Not just drinks but drinking and drugs. A lot of people who do not know her history will innocently say something like "We're going to happy hour" or "Just have a drink, it is the holidays." She knows though that, for her, the likelihood of that one drink turning into passing out at 5AM after an all-night rager are dangerously high. This is why I say there is no such thing as a former addict, as you can throw everything away with just "one".