He may be the biggest prick on planet earth but he was a kid when the girl was born, she was given up for adoption and raised by another family and I don't see where she has such a huge claim on him. She obviously isn't all that anxious to get on with the birth mother, but maybe that's because the birth mother is a forgotten footnote in rock 'n' roll history.
Let me put it this way: if I gave a kid up for adoption many years earlier, had moved on with my life and then that kid, now grown up, popped up in my life, I'm not sure I'd want them there. When you haven't seen a kid grow up, there isn't the same bond. And I guarantee that if he wasn't 'Rod Stewart', she wouldn't be making such a stink now.