Sorry to hear of his difficulties.
I've watched several old friends slowly fade away, and it's heartbreaking, as you say.
I've read a lot on the subject, as I'm pretty ancient myself and hope to avoid a similar decline.
What I've found is that physical activity and trying to keep learning new things seem to be linked with lessening the effects.
Of course, to derive any benefits, you have to be fit enough physically, and have to have the interest in learning on a daily basis.
I wish both of you all the best possible.