What a difficult thread to digest! OP I will give you my thoughts as a person who placed a child for adoption and how I think I would feel as an adopted child.
Firstly, I placed my daughter when I was 21, pregnant through rape. I don't care if you are 16, 18 or 30 placing your child for adoption is tough emotional stuff and clarity on how to handle all the ins and outs up to everyone's standards is laughable. You do the best you can at the time. I had made a decision years ago that I would never contact my daughter as I had made the decision to place her and honestly did not want her to know how she was conceived. Every child should have an opportunity to be happy with no strings or in this case baggage attached. I understand your DH may have been coerced to make the decision he did as a very young man and that can have terrible long term regrets. Unfortunately, even with that being said, as an adult you have to take into consideration that his reveal may cause more harm than good. I would suggest before he even consider contacting the family he calls a counselor to talk about his feelings he had at 16, 18 etc... He needs to get his feelings straightened out before he even considers involving his birth daughter. It wouldn't hurt and I certainly think it would help.
Surprisingly , I will say if I were adopted I think I would want to know. I wish you all the best and ask that you, your husband and someone in the adoption field sort through your husband's feelings!