Sounds like he submitted the W-9 with his personal details to his clients, hence, the 1099-NEC's.  I'd also find out how the contracts were structured, how he invoiced his clients, into what accounts the clients were directed to pay, etc.

Assuming everything is consistent with what it should be for a C corp but it's his clients who made a mistake, your client should formally write to each of those clients and explain the technicalities to request a corrected 1099-NEC.

If the only mistake was him incorrectly furnishing his personal details on a W-9, he may write to explain that the W-9 filed previously had incorrect details, provide a new W-9 with the correct data, and request a corrected 1099-NEC on account that it should reflect the legal and actual structure of the contract and payment.  I'm not sure, however, that the payors are obliged to "correct" a 1099-NEC for a prior year when they had reasonably relied on a previously submitted W-9 and the 1099-NEC did not trigger any IRS notice for TIN mismatch (with the correct name and SSN of your client).

Your last resort is to report the full amounts on the 1040 and back out the same amounts with a statement to explain the technicalities but just don't expect anyone at the IRS to read it.  The same amounts would then be reported on the 1120 instead.