CIC is effectively telling you that they are getting ready to refuse your application. That's what this letter means. Before they refused your application, they are giving you an opportunity to prove that your parent does not fall under excessive demand, meaning that the cost of their health care annually will be less than $6,500. So this is what you need to prove (i.e. that they will cost the health care system less than this amount per year). Saying you will cover these costs doesn't address the issue since everyone who is a PR qualifies for health care coverage.
What you want to do is submit evidence to show how much their current care will cost in Canada. You want to include letters from their current doctor confirming what treatment they are receiving at this time. You also want to look ahead 5-10 years and anticipate what their health condition will be then and the costs to the Canadian health care system. Once again, you'll want to support this with letters from health care professionals and also evidence of how much the treatments they will need will cost in Canada.
I'll be honest - I think you have your work cut out for you and this is going to be an uphill battle. Once someone is at stage 4, it's likely they will either need dialysis or a transplant. Both of these treatment options will exceed the excessive demand cap.
Good luck.
