In good news, I talked to my dad again yesterday. I've been making more of an effort to communicate since he ended up in the hospital. He's been back home now for about a month.
After my mom died, I think he got REALLY depressed. I think he'd mentally given up, to be honest. It caused him to spiral, which meant he wasn't taking some of his meds, which meant he gained 40 lbs of water weight in a very short time, and it ended with him in the hospital, then a skilled nursing facility, and now finally back home.
I think he's in a much better place now, getting his strength back, getting into a routine where he's doing things and socializing... But after being married 50+ years, I don't think he knew how to live without her.