Health

Tell her your concerns.

Ask about the medications he is on, can they cause neuro/psych problems?

How about some blood work to rule out an infection, dehydration as pointed out by another poster here.

It sounds like he is unhappy in the facility and his stories are angry stories.
For a person who is stubborn and strong, it is hard to be "incarcerated" in a nursing home.

What issues does he have with his care? Are they legitimate issues or irrational ones?
Maybe he is angry for a good reason? This is important because we need to know if he
actually has some points that are legit, or if he is totally irrational at this point.

I would address some of his issues one by one.
And see how he does with that.

I would not let him go on for hours with these angry rants, I don't think that is good for him.
I would say grandpa, you have told us these angry stories before. Let's talk about something else now.
Or change the subject.

Or tell him that you had talked with the charge nurse about his complaints.

I have sympathy for him. I had a serious fall some years back and had to have surgery, was on pain meds,
was immobilized, felt like a captive in a nursing home. Ran into some bad care and mistakes.
I think I went a bit nuts too. It is all very overwhelming especially for strong stubborn people.