If he was self-employed in 2023 and received a 2023 Form 1099-NEC then he needs to amend the 2023 tax return that was filed to add the income from 1099-NEC.
See this TurboTax support FAQ for amending a prior year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
