You may find a life insurance company willing to insure them, but you will find they have a time period before they will be covered, i.e. if they die within the first 2-3 years, the insurance policy will not pay out the death benefit.
You can buy a graded benefit life insurance policy, but they have to make it two or three years until it pays off. Prior to that, it only gives you the premiums back plus interest.
However, depending on your parent's health problem, he/she may be able to qualify for term life insurance coverage for a term of 10 or more years, depending on the health issue.
There must be an insurable interest between the person buying the life insurance and the insured person. There does exist an insurable interest between a parent and child so you could buy life insurance on your parent if you find an insurance company willing to insure your parent's life.
Here's how you can learn about guaranteed acceptance life insurance and how it works.