Enter the child as a dependent on your tax return. You will have to print and mail the return, it cannot be e-filed.
The IRS will pay the refund shown on your tax return, if any.
See this for how to print a tax return for mailing - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turb...
Within a year the IRS will be investigating this problem. The will mail instructions to both parties on what to do. The loser of the investigation has to pay back any tax refund based on the wrongfully claimed dependent plus interest and penalties.
