Maybe not. Thanks for introducing the related offshoot that makes the topic more interesting.

I think there's more to the story Dietary Approaches that Delay Age-Related Diseases Look in the section "Diet to prevent dementia", starting at the paragraphs starting at "Fat, Fatty acids, and caloric intake." Although some studies have suggested a link between dietary fatty acid and calories and the risk of dementia, it was further qualified that the increased risk was only among individuals carrying the ApoE epsilon 4 allele. Furthermore there are contradictory studies as well. Interestingly, fish fatty acid reduced the risk of dementia.

Wendy's does have a fish sandwich I believe. Although I wouldn't recommend eating one.

With respect to caloric intake, the risk of dementia is associated with obesity rather than the number of calories, so given appropriate levels of body fat no reason to give up the daily cheeseburger and fries just yet.

I think the old guy's problem was likely unaffected by his fast food habit, more closely correlated with age. Perhaps if he'd ridden a bike more the last few decades and the SUV less, he'd be in a better situation.