A filed application does not by itself authorize work. Approval, issuance of a visa where one is needed, admission in the classification, and authorization to perform particular work are distinct steps, and treating a submitted file as permission skips several of them.
Identify which step has actually been completed
Ask what exists today: an offer, a filing, an approval notice, a visa or an admission record. Canadian citizens are generally visa exempt for TN admission; Mexican citizens generally need a visa. A person already in the United States may have a different status procedure. Confirm the specific route and effective work authorization before starting rather than treating any one document as a universal permission.
Admission in a classification does not satisfy an employer's clearance requirement, and passing a background check confers nothing on the immigration side. Start with the offered duties, not the converted compensation total. The engagement must be prearranged with a US employer or other qualifying entity, and TN cannot be used for self-employment. The applicant must be entering to perform engineering, so a process lead whose week is mostly contractor scheduling and invoice approval may be refused even with a perfect licence.
The common error is treating financial value as evidence that unrelated duties are professional.