Yes, if the agency is genuinely her employer and the placement is a defined professional engagement. The letter must describe the specific accounting work and the client, not an open-ended right to place her anywhere.
Placement is fine; vagueness is not
Hypothetical example: a Rothesay accountant's staffing agency proposes a letter stating she will provide accounting services to the agency's clients across the United States as assigned. That description does not identify prearranged professional activities for a specific employer, and an officer is likely to refuse it. The same arrangement becomes viable when the letter identifies the manufacturing client, the financial-reporting project, its expected duration, her salary and the agency's employment relationship with her, and when the client's own engagement letter with the agency is attached.
TN permits third-party placement; what it requires is that the professional activity be prearranged and described. She should also confirm that the agency, not the client, has the power to hire and dismiss her, because if the client controls those matters the client may be the true employer and should write the letter instead.