"It was less than $600, not enough to trigger a 1099-B issuance from the provider."

Are you trying to describe 1099-Misc or 1099-K? Because 1099-B is for transactions, and I'm not aware that there is a limit for reporting. Did your taxpayer even look to see if their provider has a form available to them online, or just assume there will be no form?

Are you aware of the pending 1099-DA?