Drinking in private homes is perfectly legal for 15 year olds, and just because it is available does not mean that your child will drink it, let alone get drink.
And since it is legal I think you need to be very careful wading in and telling other parents how to supervise their child's parties.
I also think you need to be very careful wrt adding in to protect your dd. She DID tell you, and may well feel embarrassed and betrayed if, instead of using this opportunity to meet her trust with a sensible talk about how she feels about this and how she would handle any peer pressure, you call up the other parents, make a scene and ensure that this is not only the first, but last party she gets invited to.
However her history as a vulnerable young person and the context of alcohol is a pretty massive drip feed.
I would quell your feelings of outrage and lack of 'condoning' etc and just talk with her. Work out how she will resist any peer pressure, or talk about anything that might upset her. At 15 she is entering the years when friends will start to drink - hopefully only weak drinks, and do it responsibly. But from now on you can't just adopt a 'shut it out' approach. IMO.
If there is peer pressure the 'outside only' policy is the prefect get out: she can wear something thin and just say it's too cold outside.
There used t be a wise teen-parenting-MNer who gave her kids a secret code: If they texted a certain innocuous phrase the parent would send a text saying 'Oh no - have to come and collect you early because we need to take the cat to the vets, coming in 10 mins' or whatever, and the teen could use this as a way to leave early while saving face.
Give her choices and strategies...and don't over-react.