What your client should have done to avoid FIRPTA withholding was to submit a W-9 at the time of sale.

Since the IRS' schema does not permit the filing of F.8288-A with F.1040 but F.1040NR, the return may need to be filed on paper.  If you override and e-file, just be ready to respond to a notice as the IRS' system wouldn't know how to process the F.8288-A you attach as a PDF and may not be able to match the FIT.