Your client can't put the withholding through his tax return.  The certification, withholding, remittance, and reporting requirements explained in the original response are governed by different chapters and sections of the IRC compared to the filing of F.1040 and payment of the balance due on that return from the taxpayers concerned.

Furthermore, if your client decides to pay the tax on behalf of the ex, that payment most likely does not constitute deductible alimony and there may be gift tax considerations.

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