Assuming that she is single and that you are claiming her as a dependent on your tax return, she needs to file a California tax return if she made more than $4,236 (the California standard deduction) in 2017. Even if she made less than that she will likely get a refund of all the withholding by filing, in which case it would be worth filing the California tax return. She will file in California as a nonresident. She will also be required to file an Oklahoma return if her income is over $6,350. When she files an Oklahoma return as a resident they will give her credit for any taxes she paid to California, so she will likely not owe anything to Oklahoma (this is called the other state tax credit)
Sources:
https://www.ftb.ca.gov/individhttps://www.ftb.ca.gov/individuals/fileRtn/uals/fileRtn/
https://www.ftb.ca.gov/forms/2017-California-Tax-Rates-and-Exemptions.shtml#ifr
https://www.ftb.ca.gov/individuals/faq/net/902.shtml
https://www.ok.gov/tax/documents/511Pkt-17.pdf (page 4)
