I've got a lot of girls myself, but they haven't hit their tweens, so your post really hits home with me. I don't know if your doing something differently would have made things turn out differently. With my kids, I'm telling them not to date until they are old enough to get married, or just a short period of time from being able to marry. I don't plan on letting the girls go around one-on-one with boys or have my son date like that either. But I also realize that way back when, women married at 16, and biologically, it kind of makes sense for those who have reached their full height, etc. at that time. The body is in peak fertility years in the late teens and early 20's.
I wouldn't put my child on birth control. As far as pills go, they don't protect against disease, and they can actually stop a baby from implanting, so my wife and I have never used them. But I wouldn't want to do anything to encourage fornication.
Does she consider herself a Christian? If I were in your situation, in my talks with her, I'd be talking about her relationship with God.
Another thing to consider is whether this young man is the type of guy who'd be suitable-- down the road-- as a potential spouse. If he were a Christian who'd fallen into sin with your daughter and repented, neither of them slept with anyone else, and he'd be a decent husband, I'd be thinking as a parent in the Old Testament about this. The man who took a girls virginity was obligated to marry her unless she was betrothed, in which case they were to be put to death (if there were two or three witnesses.) It would help keep the young folks from violating the 'one flesh' principle even if they did fornicate.
As a dad, if a young man were doing that to my daughter, we'd be having some words about it. If he wanted to persist, I might mention the statutory rape issue. And my daughter wouldn't be leaving the house without myself, my wife, or someone we trusted. I might consider home school, moving, etc.