Payroll evidence rather than testimony: T4 slips, pay records and a record of employment covering the whole period, supported by an employer letter that states the exact dates, the duties and the reporting lines, and by any secondment records showing time spent working in the United States.
Dates from records, duties from the letter
The year is a factual question and it should be answered with documents that were created at the time. Assemble continuous payroll for the full three-year window rather than just the year being relied on, because an adjudicator will want to see where the qualifying year sits inside it. Add the employment agreement and any amendments, since a promotion into a managerial role part-way through the window changes when the qualifying period actually began.
Where Marise spent months on the American side helping with the acquisition, produce the travel and assignment records rather than hoping the gap is not noticed; time in the United States generally does not count toward the year abroad, and a discrepancy found later is much worse than one disclosed now. Finally, have the letter written by someone senior to her who can speak to what she did, not by her.