Elderly father being manipulated

I understand that in the vast majority of cases, we have zero control over another person’s choices and that our goal is to accept people exactly as they show up – but how does that work when the person is elderly and being manipulated and possibly abused? I strongly believe that my mother is putting thoughts into my (elderly, beginning to show early sign’s of dementia) father’s head to keep him away from his grandchildren. I do not believe that my dad is truly choosing of his own volition to see the kids as little as he does these days, I believe my mom is purposely getting into his head to tell him he’s “too busy” or “too old and tired”, even though what my dad wants most in the world is to spend time with his grandkids. I believe he’s being manipulated into doing things that he doesn’t truly want to do, and that he’s missing his chance to spend time with his grandkids while they’re little, and that he will regret it when all is said and done.

I got coaching on this today and the coach emphasized this it’s his choice to spend time with the grandchildren or not. But how does this work when someone no longer has complete mental control because of old age?