How do I best help my parent when their spouse of 64 years dies? How should I help them before the spouse dies, ie prepare emotionally?

It's pretty hard to prepare for the virtual amputation of half your heart and soul, but I'd say at least be sure you're honest with them about what's likely to happen if they're competent to handle it.

My dad was the healthy one and Mom had dementia, but he was the one who got sick and died first. He didn't want to upset Mom for whom he was a rock who'd never leave her, but after a couple of weeks of trying to hide it, she realized something was up. She didn't really have the competence to understand the whole cancer diagnosis, the treatments, and all, but I finally convinced Dad to let me tell her gently that he was indeed sick and not going to get better. I'm not sure how much it helped, but it would have been awful for her to be told he was gone with no warning. This all happened in six weeks from diagnosis to death, so we didn't have a lot of time to think about what to tell her or when.

Her dementia (vascular) robbed her of her emotions, so we didn't really know too well how it affected her, but we chose to respect the longevity of their relationship and let her know that their time together was coming to an end.