How do I get proof of withholding for personal taxes when withholding was done via the FIRE system?

IRS isn't recognizing withholding for a relative, and say they need proof of transmission to the FIRE system. The investment company that provided the 1042-S has a receipt, but it's not a FIRE receipt and the IRS says they must have used a 3rd party to file via FIRE. It's been like pulling teeth trying to get more out of the investment company. Is there a way my relative can request this information? Or can find out who the third party is so that they can contact them?