Because it's not part of her academic record, and that's all FERPA covers. Now, your COURT ORDER might cover other things, but we can't know that. AND, you can't force the school (a 3rd party) to comply with YOUR court order. IF there were an open court case, you could subpoena the enrollment documents.
Something else to consider - I just enrolled my kids in school in a different state than they'd lived in their whole lives. The paperwork at the old school asked for information regarding the parents. The new school requests information regarding the "custodial household" and "non-custodial household". So, at first glance, their paperwork would appear that their Step-Father is listed as their Father, when in fact, their father's information (and his wife's) is on a separate sheet of paper.
I don't think I explained that very well, but what I'm saying is that without context, or an ability to show some sort of nefarious intent on Mom's part, a piece of paper that you don't have access to isn't all that helpful.