1099-NEC is an informational form. It is only required to be issued when the payment for service is between businesses, and when it exceeds $600 or more in the calendar year, and only issued to certain entity types (not C Corp, not S Corp). Your client working for his S Corp is supposed to be paid through payroll. The customers are paying the Corporation, not your client. Your client is supposed to provide a W9 to those businesses, with the information of the S Corp on it, and that way, they won't create a 1099-NEC for that work. Your client also would issue a 1099-NEC to his own subcontractors if all of this applies, that's why you see this as a question on the tax form. If the payments are through a "settlement agent" such as credit card or PayPal or Venmo, there is not a 1099-NEC issued, because the payment now falls under the 1099-K rules for the payment entity.

That's also why your comment of, "who earns income from multiple sources in a single day" has no bearing on the Form 1040. This is what businesses do, they have lots of sources of income. That's part of the Form 1120-S (not 1120). That's their operation.

You asked about, "separating the client's expenses" and there are No Client expenses. They are working for the corporation, so there are corporate expenses that need to be tracked, handled and reported appropriately to those regulations. This is not like a sole proprietorship. If your client is a major shareholder in the S Corp, there also are limitations you need to know.

And the 1120-S had a filing deadline of March 15. If they got a K-1, you don't also prepare their personal taxes using any of the business details. Again, it's not the taxpayer's personal activity. A corporation is its own entity.

"to confirm the 1099-NEC goes on the 1040? Corect"

Never. A 1099-NEC is evidence of business activity. A taxpayer is supposed to track everything about their business. If all the work they do is for private individuals, a 1099-NEC would never even be issued. That doesn't mean they ignore that this is a business for tax filing purposes. You file the business info, not the informational form. But you should use that to confirm they are reasonably reporting all the business info to you, of course.