I'm a pretty liberal mom, but also not keen on alcohol (several years sober myself).
I was recently a little shocked when my 13 year old informed me that at a pre-party party she was given prosecco AND vodka, and the hosting mom was aware...
I wouldn't give spirits to my own teenager, let alone someone else's!
In this case, I would try to find out what the deal is with the party, who will be bringing what, what level of supervision is in place, and go from there.
But my general rule (about sex, drink, drugs, smoking, everything) is to be as non judgemental as possible and teach my daughter how to explore these things safely, because repressive parents are never successful in stopping these things, they only ever provoke their teens to try them in less safe ways. I am 100% confident that this is the case.