Generally, USCIS honors the I-130 withdrawal request immediately. If there was an I-485 associated with the filing, there will be a denial decision that will be issued. I have seen letters confirming an I-130 withdrawal in the past. You may want to confirm the withdrawal by looking up the receipt on the USCIS website. Remember, the I-130 is controlled by the US citizen or Lawful Permanent Resident, so any correspondence would go to them, and not to the foreign national.
This general advice does not create an attorney-client relationship.