She got a green card under 18 and her father naturalized when she was under 18, so she still qualifies* for N-600 even though she's over 18 now. For the N-600 they will backdate her citizenship to some time before her 18th birthday.
She can also apply for a US passport, and then seek the N-600 later.
*assuming the other conditions of the Child Citizenship Act were satisfied
EB3 ROW I-485 Approved: July 2007
USC: July 2013
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.