A 1099-K is activity and not income. For a retail establishment, it includes amounts collected from customers for sales taxes, further evidence this is not an Income reporting form.

https://www.irs.gov/payments/general-faqs-on-new-payment-card-reporting-requirements

"Separate reporting of these transactions is not required. However, you should follow the return instructions on the form you are completing to report your gross receipts or sales. You should report items that qualify as a trade or business expense on the appropriate line item of Schedules C, E and F."

https://www.irs.gov/businesses/understanding-your-form-1099-k

PayPal will likely have issued this to your taxpayer who is using the Business side of PP for their transactions. They opened this door.