You would report the sale on your client's return as US-ECI.

Your client will claim a credit for the taxes withheld on the return but must attach a copy of the F.8288-A showing the taxes withheld.  Assuming the buyer/agent followed the proper procedures, the F.8288, F.8288-A, along with the withholding should have been transmitted to the IRS within 20 days of the sale and the IRS would then have sent your client the stamped Copy B of F.8288-A, which your client must attach to the return.