Her US passport will show her country of birth which is Cambodia and nothing special but just getting into the visa line and telling them your of Khmer birth so they will stamp the appropriate K visa. And yes you can do it online by going to the Cambodian embassy in Washington DC website but that will take a month or so and cost money because you will need to pay for the shipping cost for both send and return. Also for future reference any of your kids will have the same privilege as their mother just show birth certificate as proof.
