Your daughter is a minor and so she has fewer rights than an adult. She doesn't have the right to do anything she wants regardless of the risks, just by claiming the right to privacy. I think parents who have suspicions absolutely MUST check their kids' phones, Facebook pages, and rooms!
You want to protect her privacy but you are responsible for her safety. You absolutely have to tell her you found her phone and read the message. In fact, have you considered that she might have left the phone there on purpose in the hopes that you would look? If she's never without it, it's very curious that this is the day she happened to leave it under your nose.
She might be very troubled by her friend's behavior (or her own, or at least her temptation), but she can't tell her friends that she confided in you! So it's easier to cover herself by saying Mom snooped!
Two teen sisters who lived near me were photographed at parties waving water bottles full of vodka. They put the pics on their Facebook pages, sent them to friends, etc. No one intervened. They were killed in a car crash while under the influence, and boy, do their parents wish someone had snooped and told them! The police chief says he is so emotionally exhausted from ringing so many parents' doorbells to tell them their drinking kids are dead.
I think you need to have a talk with your daughter and find out what's going on. Tell her you know about the message and that you love her so much, you are going to watch out for her. Her brain will not be fully developed until she is in her early 20s, and the last part to develop is the ability to predict consequences of actions. You have to be there for her until she can do it herself. Whether she screams at you or acts relieved, you need to pursue the discussion.
Yes, the phone is a privilege, and so is going out with her friends in an unsupervised fashion. Those privileges can and should be revoked if there is a question about her behavior. She does need to be, as the other poster said, the same person inside the home as outside.
Finally, I think you have an obligation to tell the other girl's parents what you found. Maybe the girl didn't drink, maybe she never actually went through with the plan to put scotch in the bottle (and how dumb is that, to put a dark liquid in a water bottle???), maybe it's all bravado, but they need to know and to talk with their daughter. How would you feel if the situation were reversed? What if another parent knew that your daughter was drinking and didn't tell you? What if something happens? None of you will ever recover from it.
Good luck. This isn't easy. I have a 20 year old and have been down this road. But you must be the parent here, no matter how hard it is to tell your daughter you read her messages.