I would NOT recommend unless she is prepared to either re-enter by land (USA), be away long enough to get passport abroad (4-6 weeks?), or has a visa waiver passport and prepared to risk it (somewhat) under this program:
https://www.canada.ca/en/immigratio...l-canadian-citizens-visit-canada.html#findOut
Better would be to delay departure by ~ a week. It is possible a temp passport (urgent basis) while abroad could be obtained but also a risk.
My understanding is that when you do the ceremony and receive the citizenship certificate (e-certificate), they warn that it cannot be used to apply for passport for 2-3 days. Hence why I'd suggest at least a week - esp if around holidays when passport offices will be closed at least a few days.
To be clear what the issue is: they will NOT let a PR who has become a citizen board a plane - a passport is required. Exceptions are few.