I want to make a clarification....
She has only emailed my dh as of yet.
So now what, P&W?
I don't know, how does your DH feel about this? I know, he is in shock. But, I am sure you both are talking about the possibility.
From the years of talking to you here, my feeling is that you are both level headed people and will handle this the best way possible.
Coming from my end...let's just say she is his DD and she just wants contact with him. I know that it is more common for adults to make this move than children for a few reasons. When you are a child, it's a loyalty thing. She may have felt that her stepfather and mother would be upset if she contacted your DH. Or she may not have had any interest in stirring the hornets nest. I know that I had zero interest in looking up my father when I was a child. Just enough other stuff in my life, that would have just complicated things for me. If that makes any sense.
As an adult, especially with children of her own...she may be reevaluating family and what that means to her. And if there has been a divorce, she may not feel that she owes her step father the same loyalty anymore. Or they could have an estranged relationship for a variety of reasons. Or she is just curious. Or she just finally has the guts to contact him. Who knows?
Unless you and your Dh are wealthy, I wouldn't automatically assume she is looking for financial gain. I am sure she knows she cannot force that from you anyways, legally she can't. Even with back child support issues, it would be really unlikely that she would really benefit much in the long run. Let's be real, that would definitely not make for a great father/daughter relationship either.
I am going to guess that she just really wants to know him, what he is like, what his life is like, what he looks like...you and the kids. I mean, this is a whole part of her that she is completely in the dark about. Your DH is half the reason she is here (I am just going with him being her father). That is very significant. I am sure it is hard to understand if you haven't been in her shoes. Just as it is hard to imagine his or your position, without walking a mile.
I will tell you, when I met my father, I was blown away by the similarities. Mannerisms, physical traits...it's quite shocking to know that some of this stuff was inherited.
While my reunion with my father was not a fairy tale Oprah happily ever after deal (they rarely are)...I will never regret having him in my life for the remaining years of his life. We didn't see each other very often and we talked only every couple months. But there was a connection there. Small as it was, it meant something to me.
If he does decide to connect with her (that is totally up to him, not everyone can do that), my advice would be for him to just try to take it slow and be her friend. She is raised, I doubt she is looking for a Daddy. It's most likely she just wants to know what he is about.
And I would really surprised if you both don't feel the same level of curiosity. Instead of thinking so much about expectations and responsibilities, maybe just take it as it comes and go from there. Talking to her, meeting with her, forming a relationship...none of that is forming a binding contract. He is no more legally obligated to her now than he was before he knew about her.
I would not say not to hire an atty, I am sure that is good advice. Legally, there really isn't anything you owe her. I bet it would only take a phone call to an atty to find that she cannot get back child support and i very much doubt the mother could either. Obviously, you would need to check into this.
I really wish you both luck with it, I am sure your heads are spinning.