What do I tell my mom?

Hurtbabygirl: I HURT for you. I really do. Life is never easy when both parents survive to old age. The theraputic fibbing may be the solution. It is quite likely that she will forget where her spouse is; it's, sadly, the dementia mindset.

Theraputic fibbing. "Mom, the doctor says he needs nursing care for now. We want him to get better. We'll see how he is next week/month."

What you've had to go through these last months. Hugs.

If you look up strategies to use when a person's spouse has died, you may find ideas about how to protect your mother during this transition. I'm not suggesting you tell her that! - God forbid; but in terms of dealing with the enforced separation and your father's absence there may be some ideas for forms of words that will help to soothe and reassure your mother or distract her onto other topics. If she is still partly reliant on your father and fully aware of him it's going to be very hard at first; but a good MCU should be on the ball and able to guide you.

I still hate to think what pressure you are going to be under, though.

I haven't been in your situation, but, based on what I did encounter, I found that keeping things on a temporary basis worked well. Like, the doctor says that while you are getting your rehab, physical therapy, medication adjustments, we'll do it this way, so daddy can get his rest. They will reaccess the matter, later on. We'll have to see how things progress, we are working on it, if we are patient, things will get better, etc. What I found is that she asked me a lot, because she forgot what I had already told her. So, from one day to the next, she could not recall what the reasons were. It's not that she was insistent, She just didn't know, due to memory loss. I think it depends on how progressed your mom's dementia is.

Also, keep in mind that eventually, she may not remember that they are living apart. I'd keep in mind that your dad may need this time to rest and recover.  Managing the care of a person with dementia can be quite demanding.  It may be that your dad really does need his rest.  I'd try to reassure him that she's okay in MC, so he doesn't fret. 

Hugs from here too -- My dad had Parkinson's with dementia and he lived the last 8 months of his life in memory care. He was wheel-chair bound, incontinent and couldn't dress himself, and had some difficulty feeding himself. He was a big guy too, so not an easy patient to transfer, and became a 2-person, then sometimes a 3-person assist. Is it possible you could find a memory care facility that would take them both together?
Dad was really a candidate for a SNF, but this place was so nice and new, and he was the 2nd resident to move in. I  was so happy they accepted him, bc the staff-patient ratio was better than what I'd found in a SNF, and he had a private room and bath with all his own furniture and things he liked. I was there every day, so I'm sure that helped, but really the aides were so good with him, and he had a couple of buddies there, and even a couple of the female residents who liked to keep an eye on him and help him sometimes.

Sadly, a time may come where your mom does not even know your dad

Hugs, such a difficult thing to handle.

I am glad the 'home' in my community mostly have different levels of care, but sometimes there is no way to keep a couple together.

Can you manage a schedule to bring Mum to visit Dad regularly? That is what a friend's family did, when dad did not need care, but mum had alz. One of the daughters took him twice a week to visit Mum.