If his fellowship is taxable (some are and some aren't, see https://www.irs.gov/taxtopics/tc421), it would be taxable by his state of residence at the time he received the money. Per statute, he was a resident of DC for tax purposes for all of 2019, thus ALL his 2019 income is taxable by DC.
With regard to his possible 2019 tax obligations to California, you must decide his CA residency status based on the rules outlined in CA FTB Pub. 1031, which I referenced in my previous answer. I would particularly refer you to page 6, "Leaving California". If you determine that he was a legitimate non-resident of CA for all of 2019 according to CA's published rules, he would have no tax obligation to CA unless he had CA-source income during the year.
Note that it is possible for a taxpayer to be a domiciliary resident of one state and a "statutory" resident of another state at the same time.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
