How can a girl have NDI? I thought only boys had it.

Topic: NDI: An Overview
Author: Wildin, Robert
NDI is predominantly seen in boys. However, it can be seen in girls under two genetic circumstances. The X-linked form generally restricted to boys can also exist in girls in varying degrees of severity, even as severe as the boys'. This apparently occurs as a result of skewed X-inactivation. You can read about this in Nomura's Detection of Skewed X-inactivation in Two Female Carriers of Vasopressin Type 2 Receptor Gene Mutation and in van Lieburg's Clinical Phenotype of Nephrogenic Diabetes Insipidus in Females Heterozygous for a Vasopressin Type 2 Receptor Mutation.

However, a second genetic form of NDI exists that is autosomal recessive and due to defects in Aquaporin-2, the water channel protein. In the United States, this form is rarer. Most of the cases reported in the literature so far have occurred in the context of consanguinity, i.e., parents who are genetically related.

I assume that the case you describe has no prior family history of NDI. I would say that if there is no chance that your girl's parents are genetically related, then heterozygosity for the X-linked form (vasopressin receptor defect) is somewhat more likely, but if there is any suggestion of possible consanguinity, then one should look for Aquaporin-2 mutations.