Mixed families as they are called can be a difficult experience.
I agree with you. This young lady has established a bond with her father that may be in excess of what is normal. But, she is also acting a way that at the age of 15 is dangerous to her life and to her future. It is important to know that: This is more than just having an attachment issue with her father. This is a diagnosable issue that is close to, or is, what is called conduct disorder.
She is exceedingly angry and very hurt. As a result of his hurt she is acting this pain out through disobedience. She feels unloved, and even if totally untrue; she feels it at some deep level. She compensates for this perceived feeling by acting out. And, punishment in the traditional sense does not work as feeling attention and love, even if from some boy who is willing to have sex with her one time, is worth it to her, above any restrictions that are given as punishment.
What should you do?
This is somewhat complex, as you have been placed into a completely unfair parental role. You cannot be the disciplinarian for her. Her father must be that, fully, completely with your back up, not the other way around. It is not fair (or functional) to you to have to guide this very hurt girl. She needs her father to do this. And she will not listen to you anyway, and you will be relegated to the role of scapegoat if you continue as the "heavy". Neither status is good.
Without a doubt, you and your husband, with or without her in attendance, need to be in family counseling. The professional perspective will help a great deal and a totally unified approach is what must be used if she is to be (and I mean this literally), saved from herself.
Many times children of this age will refuse to go to counseling. That is fine, as long as you and your husband are united in how she is dealt with. But, right now your roles, although somewhat good are not ideal or best. And, like iron sharpens iron you need the perspective that family counseling can give.
Additionally, your husband, I feel is being grossly and unfairly manipulated bu guilt. He too must deal with these feelings and act in his role as her father...with all the push back she will give at first, or her life will be difficult at best in the future. This reaction that he has is not unusual, but he does need to come to terms with his own feelings and the role of a father that he is called to.
This situation can be made much better, but the task will begin with your husband and you as support in counseling. If he does this, there is a good chance that even at 15 she can recover and do better. But, at this point, professional intervention is needed. Steven