I don't have any problem with most details of this thread as my own parents used to let me drink from 13, 14 (but only one glass of beer etc) - and It never did me any harm.
I do have a problem witht he fact you gave the alcohol given to 'a friend' of your daughter. I find it strange that you would even consider giving it to someone elses children, even if you have known the girl since they were your babies. I would be well annyed if my kid came in smeeling of drink, and I found out one of his mates parents had given him a can of something without letting me know why first (even at 16)
Technically, you are the responsible guardian of her when she is in your company. If she had fainted while you were upstairs, and, god forbid, choked on her vomit, what sort of situation would you be in now.
If some adult had given my son booze (illegally, I may add - as he was below age) - and something had happened - I would have killed him and wanted to know why he felt the need to give my kids stuff without my consent. The situation could have been a lot worse, and even though you may have been OK with that for your own daughter, what about the someone else kids death or hospitalisation on your hands.
And its, alright for people here to spout off about what 12 or 13 year olds get up to these days, but rules are in place to prevent under 18s getting alcohol for a reason (just like this occurance) - and those age restrictions may even be increased to 21 (simply because underage drinkers put a massive strain on casualty and ambulance crews becuase of being too immature with reagrd to alcohol consumption) - Its one of the reasons the US doesn't have the UKs sort of drink related problems in teenagers.