Ask the employer to move the start date to after the three years are complete. Applying early on incomplete experience invites a refusal that must be disclosed later.
Move the start date, not the evidence
The experience requirement is assessed when she seeks admission at a port of entry or, for the petition route, when the Form I-129 is filed. If her thirty-sixth month of qualifying related experience ends six weeks after the proposed start, she does not qualify on the earlier eligibility date. The practical remedy is to move the start and application dates until the requirement is met, prepare the experience records, and update the employer letter or contract.
Entering as a visitor to begin work in the interim is not permitted. Once admitted, TN may be granted for up to three years and may be extended while the qualifying employment continues, but the initial file cannot rely on experience acquired after its controlling eligibility date. Hypothetical example: a computer systems analyst has the required degree, but her employer changes the project scope a week before the planned start.
She should not rely on an old support letter that no longer describes the work. The sensible sequence is to revise the offer, check that the duties still fit the profession, and then seek admission or file. A rushed presentation fails when the current facts and the signed letter point in different directions.