@BeRightOnMum - as other posters have said, you DID condone the behaviour, because you did nothing about it. You also condoned your son and his friends refusing to tell you who this girl was - that is borderline negligent, in my book.
Yes, you kept an eye on her, but you had NO way of being sure she actually had taken Xanax - I doubt she obtained it legally, and illegal drugs could be anything - there is no quality control. If the drugs she had taken had had other substances in them, she could have slipped into a coma and died. In your house. And then the police would have had some very serious questions for you.
And I am aghast that you let your son and his friends get away with refusing to give you her details - be the bloody parent! You need to insist on those details now, and inform her parents. Don’t take no for an answer.
If you don’t - and she ODs on drugs again, with more serious (maybe even fatal) consequences, how will you explain your failure to inform her parents that she was abusing drugs? How will you live with yourself, knowing that you could have done more?
I hope this is not true because if it is real, it is a damming indictment of an adult’s parenting.