"it was also reported to the IRS"

As Informational. That is not anything to do with Taxes. It is part of the IRS watching for suspicious monetary movement. There is No Reconciliation of that form; that is not treated the same as a 1099-Misc or a 1099-B. If I sell a car through eBay, for $20,001, I might get a 1099-K = over the reporting threshold. It means Activity. Not tax related. There is a double-threshold for reporting or not, to your client:

A Form 1099-K includes the gross amount of all reportable payment transactions. ... Third party network transaction means any transaction that is settled through a third party payment network, but only after the total amount of such transactions exceeds $20,000 and the aggregate number of such transactions exceeds 200.

Your Client did financial transactions with a Third Party Payment Settlement Entity. That's all it means.