The question in your daughter's case is whether or not she became a non-resident of California during her absence from the state. If she did, and if she earned no 2019 income after her return to the U.S., she would have no obligation to file a 2019 CA tax return.
CA's rules on determining residency are complex, and you'll have to interpret them for yourself. The rules are detailed in CA Publication 1031. Here's a link to that:
https://www.ftb.ca.gov/forms/2019/2019-1031-publication.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
