First of all, I am so sorry this is happening to your family. Dementia of any sort is just plain cruel.
Here's a question: is it possible/allowed for you and your dad to hire a secondary part-time caregiver? Maybe even the person who was there before, if they're available. This would be (presumably) someone your father trusts and likes, who's sharp enough to notice if anything is amiss. That person could initially come along with your dad a few times until he becomes ready to let that person come on their own. Set up a day or two or more per week for that person to hang out with your mother for, say, a morning.
With free time, your father would be able to do some self-care, even if it's just a nap (I am not knocking naps!
Even if you couldn't hire that person back, maybe there's a college student in the area who needs to make a few bucks. If there's a nursing school in your area, so much the better. A lot of students would love a little mad money.
And if the place kinda, sorta doesn't allow it, there's no law that says your mother's friend can't stop by, or your dad's. And who needs to know that person (I'm talking about this student or caregiver or the like) gets paid afterwards?
Your mother is your father's whole world. Even if her illness was something else, if it's terminal then he would also need to break away on occasion.
Also, see if you can visit a bit more often. Are your kids able to write letters to their grandfather, or draw pictures for him? I bet he'd like some unexpected things in the mail that weren't bills, to tell him that all of you love him and that he matters to you.