@Sarah Huang 

"She didn't have any W9, but 1099 NEC."

Your client would have submitted a W9 to their client, for them to get a 1099-NEC. Let’s review:

An employer asks an employee to fill in a W2 for purposes of tax name, entity id, status, and withholding. At the end of the year, the employee is issued a W4. This also is sent to the IRS.

A customer asks a consultant or independent contractor for the same info, it uses a W9. At the end of the year, the contractor is issues a 1099-NEC if the total of payments from a customer are $600 or more. This also is sent to the IRS.

For example, if you are doing business returns and are not working under a corporation, your business clients would ask for a W9 from you and you will get a 1099-NEC from them if you charge them $600 or more for that year. Maybe you run payroll every month for a business, so this is what you would be doing.

And as a tax preparer, you have a due diligence to confirm if your taxpayer business clients (even Sched C filers) have met their reporting requirement to issue 1099-NEC, MISC, etc, to their subcontractors. For example, property managers providing income to the property owner/landlords also should issue a 1099-Misc for Rent paid. There are penalties for not issuing 1099s:

https://www.irs.gov/payments/information-return-penalties

You might want to look for Continuing Education topics. I used to teach a Year End Prep class for business people, and that type of class would be useful for a tax preparer, as well.