You're correct that he's not required to file an NJ tax return if his gross 2023 income from all sources is less than $10,000. But if NJ taxes were withheld from his pay, then he'd have to file a return in order to get those taxes refunded. See page 2 here:
https://www.nj.gov/treasury/taxation/pdf/current/1040nri.pdf
The same applies to his California return. His income falls below the California filing threshold, but he'd have to file to obtain a refund of his withheld CA taxes.
https://www.ftb.ca.gov/file/personal/residency-status/index.html
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
