The U.S. entity's letter, the secondment or engagement agreement if her Québec firm remains employer, her CPA certificate, proof of citizenship, and a disclosure of her partnership interest. Draft the letter and the agreement together so they describe the same arrangement.
Where the versions diverge
The usual defect in a seconded professional's file is that the U.S. letter and the firm's agreement were written by different people at different times. The letter says she will report to the manufacturer's chief financial officer; the agreement says her firm supervises her work.
The letter says twelve months; the agreement says until the audit is complete. An officer reading both will ask which is true. Fix that by circulating one fact sheet, duties, duration, pay, supervision and location, to both drafters before either document is signed, and by dating each final version.
Her CPA certificate proves the credential; the provincial order can issue a confirmation of good standing if the certificate is old. Add a one-paragraph disclosure of her ownership share in the firm and the fact that the firm's other partners will continue serving its Québec clients. Carry her passport, the final letter, the agreement, the certificate and the disclosure; leave superseded drafts at home.