Sounds more like a legal matter to me. Might want to get a lawyer or you yourself request they provide proof that you received payment for anything from them in 2019. I would think they can not prove a transaction that never occurred. It won't be on their own bank racords anywhere - at least not on the banks records which only the bank has access to.
If you receive an incorrect 1099, first reach out to your client and try to get it corrected. If that doesn’t work contact the IRS at (800) 829-1040 for instructions. You may just have to include a letter with your tax return stating why there is a discrepancy.