One thing Parkinson's Disease has taught me is how to better comfort others who are suffering. There is so much suffering in this world. It doesn't matter whether you are a christian or not - you ARE gonna suffer. It is excruciating to watch someone you love slowly die away; to lose their abilities little by little; to see the body daily weakening.
But don't turn away! It's ok that you don't have all the answers. No one does. What matters is that you care enough to be by the side of this dear one. It's enough that you don't leave them alone. Look them in the eye and touch them. They need to know they matter. You can stand by them but you can only watch as they come face to face with the hard questions and harsh realities that inevitably come. And don't neglect to share the hope and wisdom that God has given you. It is a treasure of great worth.