Because sin is reflexive? If she were speaking with her own dad then you might use it. If he were speaking to his father, it might be used. But him speaking to her dad is not a reflexive statement, except for the reflexive emotions some of us may get from this.
I'm not really the person to answer this question, but I think that this part of Norwegian I have well figured out.
I will, of course, accept corrections.
That's just how the grammar works in Norwegian. If the possessive comes after the noun then the noun has to be in the definite form.
So it is 'pappaen hennes' or 'hennes pappa'. They mean the same thing but with a slightly different emphasis (I believe it is that that former construction emphasises the noun - 'her father' - while the latter emphasises the possessor - 'her father').