This really isn't about the girl right now; it's about your husband. It sounds like he's experiencing emotions related to his biological daughter and giving her up. Seeking to establish a relationship with her, and putting it in terms of "she deserves to know," makes it sound altruistic and is the obvious solution. However, the obvious solution is not always the best solution, especially when it involves turning someone else's life upside down, as this would most likely do. A previous poster's suggestion for him to talk with a social worker or therapist is an excellent one. Maybe it will result in deciding to reach out to her in the future, and maybe it won't. But the reality is that there is no reason for him NOT to talk with a professional before trying contact again, and it may help him see a side of this that he hasn't seen before.
It doesn't sound like you or your husband knows the specifics of what the girl's parents have told her, so from what you've written I don't think it's fair to assume they have lied to her. It is clear, though, that your husband's attempt to reach out via Facebook was not welcomed. Perhaps in the future that will change, and now that they know he has a FB account, they know how to contact him. As difficult as it may be, your husband needs to recognize that another attempt to contact them or her could end very badly. Seeking the perspective and advice of a professional could really be beneficial.