I can not speak from experience, but maybe explain to her that's mum's an dad's come in all forms and they get to be parents in different ways, but the REAL meaning of Dad is your DH. The one who loves and cares for her, and will be there forever!

Maybe even explain to her that her and her dad's relationship is even MORE special because he CHOSE to be her dad, unlike someone who it just 'happens' to. And that how lucky she is, out of ALL the little girls in the world, she's the one he wanted to look after.