You are not a witch but maybe you should just encourage her to send a text in the future if she makes the same decision just so you are aware..
My daughter regularly brings one or two friends home and wouldn't dream of asking us whether it is ok or not. It is her home and we leave it to her to make the right decision.
Your au pair is 23 and it is also her home, and on this occasion she made a judgement call that she thought would be OK. Her decision making powers were obviously influenced by a bit to drink so I do think you need to cut her a little slack.
I guess she's pushed the boundary this weekend because it is her home as well.. We know how our own children do this, as we did when we were young, free and single.. The previous weekend it had not been seen to be a noticeable issue with her friend being there all day - so in her mind it would work again.
She feels comfortable in your home - and in her home, to have offered this to another friend as a solution. So she must feel that you are an accommodating family. She must be happy obviously because she's agreed to stay another year.. but her natural character would seem that this will continue to be something that she'll feel she many want the freedom to do.. If you like her friend and they cause no fuss, then why don't you just say to her that from now on it's fine to have her friend stay on a weekend if she lets you know beforehand, and that they keep a low profile so don't take over the house..
I often read on here about the many rules that are written down for many family's au pair.. Most of our au pairs have stayed a long time but move on because they can't stand the feeling of not having their independence.. In most cases the girls have already lived by themselves at university..
One horrible alternative ending to this story could have been that on Sunday the police call round to speak to your au pair for a statement after her friend was raped while trying to get home by herself. If they were still hung over at 6pm the next day then they must have been pretty drunk, so they were looking out for each other doing what they did.. You don't mention if her friend would have needed a taxi home, or could have walked.. but either way in that state it makes one or both of them easy targets for drunken lads out late. I agree with StillSquiffy about having a cab fund - but again I'd prefer my daughter to be getting cabs with someone else.... Not great for young girls with faltering English and obviously a stranger to this country, being picked up by bogus minicab drivers..