Make sure she used contraception. If she did, explain to your husband that it could have gone worse and even though her actions were wrong she was at least mindful to an extent. You also need to look at how close she was with her boyfriend and what was their motive (make sure she wanted to do it, was it out of curiosity or something else?). I'd also recommend discussing this issue with the boy's parents and the boy himself. Good luck with this issue.
You probably don't want to hear this, but right now you need to focus first on educating her about protection. She needs to be on the pill and educated about condom usage at all times. You need to be sure she isn't currently fearful that she is pregnant. You also want to be very careful not to over-react. If you do, she will continue to do what she wants and just never talk to you again. To keep your dialog open needs to be the most important thing. After this you can discuss the disadvantages, including being hurt when she becomes attached, and how relationships don't grow interpersonally when sex is involved, because the relationship becomes all about sex and not growing as a couple. Above all else I'd avoid acting angry and instead act appreciative that she has come to you. Being concerned is fine and honest. Yes, she is young, but being sexually active at her age also isn't rare in any way.
honestly just let her be and be glad she isnt pregnant