What a kind friend and boss to ask. This is a rough question bc people with dementia often cannot see their deficits due to the disease. If you feel comfortable suggesting a doctors visit, maybe that would work. A lot of causes mimic dementia and are treatable. As a people manager, I would advise you act with caution. Perhaps go through HR?

May I also suggest you pose your questions on the caregiver main page? It gets a lot more traffic (I only saw your post bc I was alerted as the original poster). Try here: https://alzconnected.org/categories/i-am-a-caregiver-(general-topics)