Not until the petition is approved and she is admitted in L-1 status. As a Canadian citizen she can present the approval at a port of entry rather than obtaining a visa stamp first.
Approval, then admission, then work
Hypothetical example: a Quispamsis firm's board has already announced the director's U.S. start date in a client newsletter. The petition has not been filed.
The sequence is filing, USCIS adjudication, then admission at a port of entry with the approval notice, at which point L-1 employment may begin. Working in the U.S. role before that point, even remotely from a U.S.
hotel, is not authorized. Canadian citizens do not need a visa stamp, which removes a consular step, but the petition itself still has to be approved. The company should file as soon as the year-abroad evidence and the U.S.
operating evidence are complete, consider premium processing if the date matters commercially, and tell clients that the start date is subject to immigration approval rather than presenting the announcement as a certainty.