You’re not her friend. You’re her mother. Big difference. Tell her she can see her “boyfriend” at school and that he’s not to be over. When he’s at your house, he has access to her. He can get her knocked up whenever you go lay down for a nap or have an errand to run or turn your back for 30 minutes.
If you’re afraid she’s having sex. You need to take her to a medical physician and have them put an IUD in her arm or give her the needle. Pills and condoms aren’t the best choice because most teens are reckless when taking them and end up knocked up anyway. Your daughter is obviously not going to take that step. But you as a parent can make her do it by taking her to the doctors office and forcing it. You’re the parent.