Watching Sarah crawl up the stairs made me wonder once again why so many of these people don't install a second handrail. Overweight or not a rail on each side is safer and they can be bought in different lengths at most Do It Yourself type outlets.
My brother, who had MS, used a two rail staircase and descended backwards for safety while gripping the rails.
Amazingly mobile for her size. Another ten years of that weight pounding on her knees and it would be a different story.