Sounds like she used her NZ passport when leaving NZ and showed the US passport when entering the US. She also might had left the US with the NZ passport being used for checking-in/buying the ticket. This method is recommended by many dual citizens, but I've always been skeptical about it. The problem arises when buying a round trip from the US to the country of second citizenship. I blame the airlines.
This problem is even more exposed when traveling between a third country and the country of non-US citizenship.
I've been using my US passport everywhere, including the EU, where I have the other citizenship. I often travel to Japan and, ofc, use the US passport to enter. However, with the new EU electronic authorization rule, airlines will demand to see if I'm authorized to enter the EU (from Japan). Using a different passport when leaving Japan could easily create an overstay there, so that's not an option. If worse comes to worse, I'll travel to the EU via a land border port of entry, excluding the annoying airlines from the process.
Also, for anyone who may be interested: Using the non-US passport may make it difficult to obtain a TS security clearance if you're applying for certain gov positions.