Hi Moonlight - you have my sympathies. You have definitely done the right thing.
First of all your DD sounds very similar to mine - very honest. I have a 16 year old daughter and last year (when she was 15) she wanted to go to a birthday sleepover with about 18 girls - most of whom I know and are really lovely girls. My DD was honest enough to tell me that there weren't going to be any adults there. The birthday girl lives with her single mum (both very lovely) and the mum was sleeping elsewhere for the night and leaving them to it.
I was emphatic about the fact that she could not go - I was very worried about them all being left to their own devices.
Like you, I felt terribly guilty because my DD has told me which of her friends drink and/or smoke. My DD thinks it's decidedly uncool to drink/smoke and I couldn't explain to her my reasons other than explaining that it was simply a feeling of being very uncomfortable about the sleepover. She too protested greatly and said "you know I won't drink/smoke" etc. but I did not give way and was accused of being the most horrible mother etc.
However, after the event my DD was fine with me and I realised that she was actually a little relieved, although she would never admit to it; but I think she was secretly worried herself, and I wouldn't be surprised if your DD is secretly worried too - there will be an awful lot of peer pressure already, without adding boys to the scene - that can only pile on the pressure to do stuff you wouldn't normally do IMO.