No. A pending application does not authorize employment. She may keep her Canadian job and visit the United States for meetings within visitor limits, but taking up the sponsor's role requires either permanent residence at the end of the process or a separate nonimmigrant work status such as TN or L-1 if she qualifies.
Bridge with a work status of her own or wait
The timeline question usually hides a status question. If the sponsor wants her in the United States before residence is granted, the only lawful routes are the existing nonimmigrant categories. A TN is possible only if her role fits a listed profession with its own qualification rule; a sales executive rarely does, though a role that is genuinely management consulting or accounting might.
An L-1 is possible only if her Canadian employer and the US sponsor are related as parent, subsidiary, branch or affiliate, which a customer relationship is not. Absent either, she waits in Canada, continues her Canadian employment, and visits for meetings, trade shows and negotiations as a visitor without performing the sponsor's work. Record each entry, its purpose and its length, since that history will be reviewed at the immigrant stage.
If the sponsor pushes for an early start on the theory that the pending application 'covers' it, that is the point to stop and re-read the official terms together.