The manuals say that people with AIP should generally be allowed to re-enter Canada if there are no problems with their PR application. Once you get AIP, the risk of being denied entry is supposed to be low. The danger is that if you are denied entry and not let back in quickly, then you lose your application, because the requirement to live with your spouse in Canada until PR is granted can't be met if you can't even get into the country. In that situation, you'd have to re-apply outland.
"Implied status" is another thing you get only when you apply inland - even if your work permit expires, you can automatically keep working under the same conditions until a decision is made on your PR (provided you submitted an OWP application with the PR application). There are some exceptions to this, it seems, so you'd have to check whether this is applicable to a postgrad work permit. If so, she'd be able to continue to work in Canada until a decision was made on the PR.