It's unclear to me what your goals are, and this is the first step in setting up guidelines or procedures of any sort.It sounds from your message as if you are conflicted with regard to your goal.I will speak from my experience as a troubled teen.The most important thing you can do (and obviously already are) is to be involved, and concerned.As for the rules themselves, I think that the rules we set for teenagers are a safety net, not a protective coating, the kids can get around them if they are determined.Set the rules that you think are appropriate for her, if they are not 100% enforceable acknowledge this to her, and be clear about the consequences if you do find out that they have been broken.
Talk to your daughter about different things that can happen when she is with him and let her know she can make a choice about what she wants to do. I also tell her if he wants to ''go farther'' than she does, she absolutely has the right to say ''no''.
(There's a chance that he's a friend of a friend of a friend!
My mother-bear self wants to lay down the law and tell her no way in hell is she going to be dating an 18 year old who we know nothing about.
I want to support her to start thinking through some of this for herself, but whereas she used to have very good judgment, these days she's running on low self esteem and hormones and I believe would follow anybody home who told her she had beautiful eyes. I talked to her about birth control and safe sex and she clearly was not ready. She went to his house once or twice, all when parents were home. When she has been out later with others I often pick her up (the joys of cell phones! I guess when there is another boyfriend I will want to meet him also.
( And yes, we've been addressing the low esteem for years.)She has no experience with dating. I say I could get behind that better if he would stop commenting on her physical beauty and if she would stop gushing. When she saw him I kept my cell phone on and when they were alone for the first time and he was pressuring her for sex, she called me and I picked her up immediately. I think it's a matter of what you are comfortable with and what she wants too.