And to even be eligible for the free ride N600/N600K, she should have been under 18. That's why I mentioned I-130 too.
Additionally, for her to qualify for N600K, the parent and her should have been living together abroad, not separate. So, based on what you said, she doesn't even qualify for N600K due to age + physical custody (we've already ruled out N600).
So, I believe the correct path would have been for her parent to file an I-130 to get her to US. Then proceed from there.
@Mike E will be here shortly to confirm or fix any errors, if I made any.
Going by your original post, the only way I see that she got N600 is because she answered questions wrongly on the online application by stating she lives in the US. Now oath has been scheduled, how's she gonna attend? She cannot come to US without a proper visa and by the time that visa is granted, oath would have been long gone. And she's above 18 to qualify for N600 anyway.
And NO, N600 certificate cannot be mailed to you in lieu of you appearing at interview.