Yes, provided the firm or the client is her employer and the letter describes the placement truthfully. TN does not permit self-employment, so the arrangement cannot run through a corporation she owns, and the duties must be engineering rather than mainly administration.
The structure of the engagement decides the employer
The Engineer listing is satisfied by her degree or licence; the harder point is who employs her. A consultancy that hires her onto its payroll and assigns her to municipal projects is the employer, writes the letter and directs the work. A consultancy that wants to bill her as an independent through a numbered company she controls is asking her to be self-employed, which TN excludes.
There is a permitted middle case: a Canadian employer sending its engineer to render services to a U.S. entity under a contract, with the U.S. entity's letter describing the services.
Whichever structure the firm has in mind, the letter must match it, name the place of work and identify who supervises her. Ask the firm directly, before accepting, whether she will be a W-2 employee, a contractor, or an employee of a Canadian affiliate on assignment. The answer shapes the letter, the tax position and the renewal.