I have a family member who married a woman who had an infant. The infant's biological father was not in the picture at all and never would or could be.
My family member decided that the infant would never know that he wasn't the dad. He legally adopted the child (when the child was too young to know what was happening) and it was never spoken of again. The child, and his siblings that would follow, never knew that he had a different biological child.
Many people encouraged my family member to celebrate being his dad - to have an adoption day, or at least to talk privately with the family. But instead he went to great lengths to hide the truth. He ignored all the wisdom that was given to him.
When the child was 11, a relative came to visit who hadn't seen the family in years and years happened to mention how cute the child was at his parents' wedding. He was only 1, and wore a sweet little tux. He asked how was he at the wedding (my family member had forbidden any wedding photos to be displayed in the home). They told him the truth.
He was devastated. It was a horrible way to tell him, and it was done badly. They stumbled around an explanation, and he realized he had been lied to his whole life.
The child later said that he wished his parents would have talked to an expert about how to correct a lifelong lie. He wished that they didn't try to tell him the way they did - just fumbling for words, trying to explain the coverup, assuring him they only had his best interests at heart - and wished that they had gotten counseling, that he could have spoken to a counselor. They never spoke to a counselor or pastor about it, and he never was given the chance to work through such life-changing news with a professional. Today, he's in his mid-twenties. He doesn't trust people, or himself. Frankly, he's a mess. He seeks girls who need him, not for something helpful, but to fuel their alcoholism, or to babysit their multiple children who have no dads, or to pay their rent.
He has admitted to his mom (he's estranged from his dad) that when he found out by accident that he was adopted, and when he was told badly by his parents without any professional guidance or support, that it changed something important within him. Basically they told him that it was true (what his relative told him) and he didn't have the same biological dad as his 4 brothers and sisters, but he was told "not to stress about it" and "hey we're still all family, right?" with a big old stupid grinning happy face.
So I don't know if this helps, but it's a similar situation, directly from a child's mouth who went through this whole thing. Bottom line - the child wishes to this day that either his parents had gotten some professional advice about how to bring up such a sensitive subject, or that he could have worked through his feelings with a therapist. It's a huge thing to find out.
I'm sorry that your husband is so cold and thoughtless and hurtful. But your daughter doesn't have to have her life ruined - as long as such important news is delivered properly and with counseling. Regardless of whether you get back together, your daughter deserves truth and honesty.