Just had another thought. You mentioned that sometimes he will just stop and stare right in the middle of things. This year I had to learn about seizures. It turns out that there is a kind of seizure that is not uncommon with dementia. They are called "absence seizures" Here's a segment of the Google AI summary:
"A brief, blanking-out episode is medically known as an absence seizure (previously called a petit mal seizure). It involves a sudden, brief loss of consciousness where a person stares blankly into space, typically lasting only 5 to 15 seconds.
- Key Signs & Symptoms:
--Vacant Stare: The person abruptly stops talking, moving, or responding to others, appearing to daydream.
--Subtle Movements: The episode may include fluttering eyelids, slight lip-smacking, or minor chewing motions.
--No Memory: The seizure ends as suddenly as it began, and the person usually resumes their activity with no memory of the blank period."

If this fits your friend's behavior it may be an "in" for you to get him to a neurologist. In other words, you would not be talking about what he considers vague arguable symptoms, but somethign as clear and scary sounding as a seizure. Seizures of all kinds take a toll on the brain every time they happen. This can clearly accelerate the normal decline.