It would not be surprising if Barb outlives Chris. Many studies show that autistic individuals typically die before 40.
Pretty much all studies show that Autistic individuals have a dramatically shorter life span than their neurotypical counterparts with average life spans as low as 36 years in at least one study and 39 in another.
Here is an article about life expectency and ASD written by a 36 year old autistic individual who like Chris mentions stress as a huge factor:
https://www.vox.com/first-person/2018/2/19/17017976/autism-average-age-death-36-stress
another article about said topic:
https://www.psychologytoday.com/gb/...early-death-in-those-autism-spectrum-disorder
If Chris does outlive Barb, there is a very good chance he won't live much long after given his dependence and reliance on her to at least be somewhat functional. It is pretty clear that because of his parents piss-poor job of raising an exceptional child, that Chris will not be able to function independently. Without quick intervention soon after his mothers death it is likely he will soon follow her to the grave.
I would be surprised if Chris made it till 50. Stress is a real killer, it kills millions a year through strokes and other Ischemic episodes like TIAs.