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You may approach the subject with her by discussing appropriate choices and behavior. Try to find out if in some way she feels a peer pressure to date or be liked. You are correct that she would do what she wants especially if you try to prevent her from dating. It seems that your concern is not so much about her socializing with him but rather the physical component of their interaction which could potentially lead to sexual intimacy. Try to speak to find out if she had thought about it and what may be some of the consequences for this if she does not wait until she is older. Her hormones are also affecting her behavior and you may reassure her that you know what she may be going through. At her age, she is going to be egocentric thinking that others (adults) do not understand her.
You would want her to trust you and be able to come to you as a confidant. You may hold of on telling her that you read her journal. If you believe that she would like to see this boy, and that some sort of a compromise may be reached, you may encourage that he visits her in your house where you can monitor their interaction more closely. You realize that ultimately she will try to do what she believes is going to make her happy as you are not watching her 24/7. At this stage, you're educating her and also assisting her in realizing what is age appropriate behavior and what can be some negative outcomes of going further with physical intimacy ex (diseases, pregnancy, regret, being labeled, being emotionally and psychologically immature to deal with the situation, etc)
Yes, as a parent you would always be concerned. Yet, for the time being it would not be healthy to think that something else had happened or imply to her that it had. You're trying to establish a good trusting relationship b.w the two of you where she would feel comfortable enough even if something took place or if she is thinking about it. A lot of teen behavior is due to peer pressure (if her friends are doing something that she feels she has to catch up to) And you would speak to her about self worth and what self esteem means to her.
You would try to encourage her to share what her interaction with her had been so far and you can still let her know that you'd feel more comfortable having them near by. You can let her know that you realize that girls her age may be experimenting in their relationships and that you are making sure that she is not doing something she'd regret.