If I understand it correctly, she will be changing her last name to her husband's last name. If the application started with her last name (before getting married - Maiden name), then her application is being processed under that maiden name. If she renews her passport and adapts her husbands last name, then you are now dealing with two different last names, and also remember, that in the application you are specifically asked what travel document you will be using, which also contains your current legal name. This may well affect the background checks, therefore may cause further delay in the processing.
I hope others with the same or similar experience weigh in or share their experience as well. I may have a similar case, since my wife wanted to renew/replace her passport (not expired yet) to adapt my last name, but since passport renewals these days are taking longer than usual due to effect of COVID lockdowns/restriction on embassy services, we decided not to renew/replace it yet, and use her existing passport, and will just worry to change her last name, once she land in Canada.
I suggest,
@eb.hans, that you renew your passport right away, since you will be needing a valid (at least 6 months) travel document for you to travel/land in Canada, but don't bother changing the last name yet, as it may complicate the application and may cause delay in the processing.