MrsYoungie, I sympathize with being caught off guard. It sounds like you are really against your 15 year old drinking, so perhaps next time being firmer with your 'no' would be appropriate. If she wants to let her kids drink, it shouldnt be in your house (yes, you are liable for that, just ask the parents that held a post-prom (read:drinking) party at their house, who were then arrested. Yes, they took everyone's keys, but underage is underage). A simple "no drinks for the underage people" should suffice, and if she presses, a "if you would like your children to have alcohol, please do it in your own home. I cannot condone those actions or accept liability for them.'
If you are worried about your child drinking at her house, ask her, and ask your child. Have a talk with both of them - separately - about what is appropriate. She should abide by your wishes, and if she doesnt, I suggest monitoring your son's contact with that family more closely, and limiting it, if it goes too far.
I am glad you continued to have a good time, and whether she thinks you are a prude or not, dont be afraid to have a backbone on this one. Good luck!