If it helps you, one of my friends wife had the same issue. During application we found she had cancer, she underwent surgery and is recovering. Their family became PR. Canada did not even do a follow-up furtherance. Biggest difference is they were in-canada applicants. The guy had a heart issue and underwent surgery one and a half years back and wife was found to have cancer just before application.
Canada's goal is to limit burden on medical system... It seems conditions like kidney issues and heart issues are more troublesome because they have very predictable degradiation and demand. Cancer is a very different beast.
It sounds like what's different in this case was that it was a spousal sponsorship, not a parental sponsorship (if I understood correctly your description). "Excessive demand" exclusion criteria is not applied for spousal sponsorship (nor of dependents i.e. children). Basically they only apply medical criteria that would amount to public health/danger to public issues, and most of those are handled by treatment and follow-up (eg communicable diseases).