My two cents:
You need to know where her head / heart are at regarding her absent father and react accordingly. If she thinks he was a missing knight in shining armor, tread carefully. If she despaired him, be more honest and open.
Tell her it must have been tough to grow up without a father like that and then let her talk about growing up. Pay attention to her responses about her father, other male influences. React accordingly.
Under no circumstances would I openly bash him. Even if she says she hated him for abandoning her. Be gentle and good luck,
Tell her it must have been tough to grow up without a father like that...
I’m assuming she did grow up with a father? This doesn’t say she didn’t. Hopefully he was / is a great guy, and this visit will satisfy some curiosity. This sperm donor was never her father.
PR’s advice is good.