If you could, you might want to have a goal of get teen to stop having sex.
But you seem equally convinced that this is not achievable.
When my daughter asked me when I thought a girl was ready to have sex, I replied ''when she's ready to handle a baby.'' We all know that every form of birth control has a failure rate, so I don't think it's too far out to talk about pregnancy.
A lot of oral sex goes on in dating, with definite health risks, loss of reputation, etc.
I find myself in the unsettling position of wanting to say very conventional things like ''An 18 year old boy only has one thing in mind.'' And then I feel bad. Older guys pick on younger, more vulnerable girls because it's harder for them to say no. They mostly spent time together at school, at lunch, or met before school for coffee. I told her that I wanted to meet him before they went ''out'' anywhere. Just make sure she knows that she can call you anytime and you will pick her up. That way she might talk to you about any things she doesn't know how to handle.
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''.
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.
If you are not condemning (and sounds like you are not) but instead acknowledge the fact that this is ultimately a choice she must make about her own body, she will be more able to hear the things that you want to say about the situation.
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.
(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.