It heavily depends on the circumstances to be honest. My mother and father split when I was two years old. She told me around the age of 5 or 6. The way she explained it was that my dad was the man whom she was married to and was around me all the time; while my father was a man who lived in New York and contributed to my birth.
I believe she explained this to me for one of two reasons, as I haven't asked her why she did it, nor do I care to ask either.
- My father wanted contact with me.
- I asked.
To top this off, I also see it from the perspective of a father who cares deeply for his children. I have a total of four children; three with my wife, and another from a previous marriage. His mother refuses to allow me to have contact with him, regardless of court orders. I've tried many times, but the point of the matter is; I care how she may break this news to him. This is why I say it heavily depends on the circumstances.
If her father isn't around (for any reason), and won't be around (for any reason); then there is really no reason to tell her, and if you and your husband are the only people who know (aside from her father), then there isn't anything to worry about.
On the other hand, if her father is around, but just now wanting to show interest (for whatever reason), then you should truly take care in how you break the news to her. It's important for your husband, and her father to get along together (truly all three of you should get along, without quarreling of any kind), prior to introducing this topic in any way. That way she can generate her own opinion of her father, instead of it being placed in her mind based on what she sees or hears.