If she would have returned at the time of your first post, before she reached 3 years outside Canada it would not have been a problem because she still would have met the residency obligation.
I am not sure why you would wait so long to ask for more advice on this issue because now she has lost that chance.
Now that she no longer meets the residency obligation with over 4 years outside Canada, the question is if her PR card is still valid.
If her PR card is still valid, she can return to Canada and take her chances with the immigration officer.
If the immigration officer does not report her for being in breach of the residency obligation, she would enter as still PR and would be able to put her PR status back in good standing by staying for 2 years straight before she applies to renew her PR card. She can apply for CPP though because that is not connected to immigration.
If the immigration officer does report her, she can appeal for her PR based on her medical condition. However, I would not give her very good odds of winning an appeal because she could have had medical treatment in Canada and as a PR, she has access to welfare, disability and everything else that Canadians and PR's have and so she would not have been in a worse position than any other sick or disabled person who is without family or estranged from their family. However, she could stay in Canada during the appeal processing which can take 1-2 years.
If her PR card is expired, she could appeal for her PR by applying for a PR travel document from the Canadian embassy explaining her situation. Again, I don't think she has very good odds but you can try.
If she loses her PR, you can apply to sponsor her but it's been made harder to sponsor parents these days. You have to show that you have income to support her, there is a small quota of applications for each year that fills up fast, the processing takes years and if she has medical problems that are considered severe and costly for health care, she can be denied based on that.
