Her risk is that she fails to meet the residency obligation and her PR card is expired.
1) If she is visa exempt to Canada, she can board a plane on the strength of her visa exempt passport. When she arrives in Canada, she will show her expired PR card and say she had a family emergency and did not have time to renew before leaving. Immigration will let her enter. They can however if they suspect that she did not meet the 730 days / 5 years residency obligation report her for that and she would have to appeal for her PR.
2) If she is not visa exempt to Canada, she can not board a plane without a valid visa or travel document. If she can get a US visa, she could fly to the US and enter Canada by land. Immigration will let her enter. They can however if they suspect that she did not meet the 730 days / 5 years residency obligation report her for that and she would have to appeal for her PR.
3) As in 2) except she can not get a US visa, the only option now would be to apply for a PR travel document. If she does not meet the 730 days / 5 years requirements and she does not have solid H&C grounds to explain at least enough of her absences to cover that time, she will be refused and her PR will be revoked.
As long as you meet the 730 days / 5 years residency obligation, there is no problem travelling, even with an expired PR card because you can apply for a travel document and you would get it. It does however take a few weeks to get one at some embassies so it can delay your return. Therefore, it is advisable to avoid the hassle of having to apply for a travel document by renewing your PR card before it expires.
I assume that the friend mentioned in the post did not renew her PR card because she did not meet the residency obligation which is the correct approach in that situation. If somebody has stayed outside Canada too long and does not meet the requirements but they are able to enter Canada without being reported, they can put their PR status back in good standing by staying for 2 years straight, even if the PR card expires in the meantime.