Be aware that your client could be subject to a penalty for not filing a correct 1099.  It is also hypothetically possible your client could be subject to paying the 24% Backup Withholding for that payment.

Make sure your client knows that he needs to receive the W-9 from the contractor BEFORE he makes any payments.  If he does not receive the W-9, he is required to withhold 24% as Backup Withholding.