It was over 20 years ago when he was first diagnosed, wasn’t it. If she has the same thing, it is not following the same pathology.
There are subsets of Parkinsons Disease. There is a set of conditions simply labeled Corticobasal Degeneration Syndrome which has many presentations of symptoms. People can and do live 20 years with some of these conditions. Even with a diagnosis of Parkinson’s, there are individual variations in symptoms/presentation.
Don’t discount the effect of will on longevity and endurance.
We are bombarded with cautionary warnings about the fragility of human life, but modern medicine can and does rescue people every day from many conditions.