Hello and thank you for your question.
I understand that you don't want to tell that it was your other daughter who informed you. Obviously you want to keep the lines of communication open with both daughters and if by telling it was her, that may make one or both of them less willing to share. So, while I don't advocate not telling the truth, but when it comes to protecting your children, then you certainly do not have to disclose that it was your other daughter. It's important that you do confront the 18 year old with her behavior as it's quite risky.
She sounds like a great kid and still is, who simply made a mistake. I would tell her that you found out through someone who is friends with one of the boys, and leave it at that. How you know is not really your daughter's business quite frankly. But you can tell her that and then let her know why your behavior is risky and dangerous, and then give her the appropriate consequence, whatever you feel would be beneficial for her.
I don't know if you need the school psychologist for this, as it could just be a one-time experimenting with alcohol, etc.
However, keep a close on her and her whereabouts and if this continues in any way, then an appointment with the psychologist could be helpful. For now, just talk with her and keep the communication open... Let her know that you are so proud of her for who she is, but that this behavior does not mirror her normally sound judgment, puts her at risk in many ways, and then ask her why she lied to you, and what she was doing ,etc. Then do keep an eye for any more behaviors such as this. Please click ACCEPT button for this information. Thank you.