My friend said dementia 100% is preventable with a whole food plant based diet?

I have a good friend who I’ve being talking to for a few years online. The other day we ended up on the topic of health and dementia/alzheimers and I told him about how my grandma succumbed to alzheimers back in 2020 and then I said “I pray we find a cure or treatment for it soon” then he said “lol there is, it’s called a plant based diet” then I said even healthy people get it and he said they didn’t eat plant based and then he said it’s all a lie and how the clinical trials haven’t worked and then I said it’s because by the time people have symptoms, the brain has already atrophied and brain cells have been lost and he said bullshit. I ended up leaving the conversation for the night.

This guy also is working on becoming a doctor…

Is what he is saying true?