I'd be skeptical about her assurances that she is going to wait until marriage for sex. I'm worried that she is telling you what she thinks you want to hear. My DD had her first boyfriend when she was twelve, and she told me then that she felt she was ready to have sex. Fortunately the opportunity with this boy did not present itself, because he moved away to a distant state shortly after they started going together. In her case, this boy lived in the apartment complex where she had a friend. She only saw him when she was visiting this girlfriend, and the mom was a stay at home mom and supervised very closely. We were lucky. She didn't have another boyfriend until she was sixteen.

If this boy lives far away, you don't have very much to worry about. But I would make certain any contacts with him were very public and very closely supervised. She is only twelve.
JoAnn, daughter 21