One of the things the big book of AA suggests is to let your friends know what your problem is so they understand why it is dangerous for you to drink. This is mainly so they will not pressure you to drink but it has other benefits and can become a part of your defence against the first drink.
I was in a bar a few weeks back, with friends. I ordered a non alcoholic cocktail. A friend who knew I was alcoholic and therefore could not drink safely, observed the barman out alcohol in the drink and let me know before I drank any. Close call. I couldn't even smell the alcohol.
I have heard many stories where a friend or family member has stepped in and helped an alcoholic who, in a thoughtless momement, was about to drink. If they hadn't known about the alcoholic and his/her dire need to stay away from drink, who knows what would have happened.