You will have to print and mail your tax return claiming the dependent on your tax return, it cannot be e-filed. The IRS will process your return as received.
See this TurboTax support FAQ for printing a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turb...
The dependent will have to file an amended tax return and change their dependency status.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
