How do I help my mother understand she cannot go back to her husband?

So sad that your mom is going through this and grieving the loss of her marriage. Has she been treated for depression? That might help with her emotional pain.
We often suggest watching Teepa Snow videos to learn how to distract and divert those who are distressed.
We talk about therapeutic fibs where we bend the truth to make life more manageable for our mentally challenged.
At this moment in time you have the CoronaVirus that you can blame everything on.
I would explain to her that we are not to travel. It’s too dangerous. That the Coronavirus is ruling our lives. Blame everything on CV as long as possible. Since she shouldn’t be going out she shouldn’t be tempted to buy him things. If she is fretting you might encourage her to write him a letter instead. Don’t argue or try to reason with her. Don’t set yourself up as the bad cop. Put that on the Coronavirus.
“Yes mom, I know you miss dh, it’s sad. If only the CV didn’t require us to stay home. Could you help me fold this laundry? I’ll do the towels and you match the socks”. While the virus might be stressful for her as well it probably would be less so than her desire to return to Florida. If the CV is too abstract for her maybe try substituting The President said or The Governor said or the News or The Doctor. So many people to put it off on. You know your mom best and what might work. You might even say DH doesn’t want you to get sick and get this horrible flu. She can’t accept the idea of him rejecting her so skip that part. I’m sorry for this extra stress in your own life and do realize that many many people grieve broken relationships regardless of dementia. In other words it’s an honest reaction to a broken heart. You can’t protect her from that. You can try to distract her for both your sakes. Call her doctor for an antidrpressant if she doesn’t already have one.

I may get flamed for this, but can you tell her that he died?

Ignore Shad.

Your profile says husband suffers from Dementia so good reason why he shouldn't be caring for Mom.

Seems hubby has found a new "squeeze" and his wife is now a bother

My mother who has dementia also convinced my dad to give up his FL home and move back to WI full time so she'd be close to "family." Entire family lives, works and runs businesses in WI.. All I ever heard growing up was how dad wanted to live in FL where "it was warm." I feel that her wish to move from FL led to his eventual declining health and death. Mom doesn't remember "dad" is gone and there is nothing one can do to make dementia patients "remember" or "understand" anything. You are dealing with someone who has the understanding of a pre-schooler and who cannot learn anything new.

Having seen the toll of living with a dementia patient took on my dad, I can understand your mother's spouse's attitude. He sees a limited number of years left and he had no life with your mother. She was a stranger in the household with increasing demands and erratic thought patterns. That will wear on anyone. You may need to find a memory care facility for your mother eventually.

Depending on the severity of her cognitive abilities, you may not be able to convince her of anything.

Come up with a vague answer to her questions and try to distract her into something else.

I’m so sorry. Since your mother has Alzheimer’s, you will not be able to convince her of anything. People with this disease become obsessed with things, as your mother is obsessed with returning to Florida. If she did return, she would then want to go back to Vermont. She has deluded herself into believing she has a husband who cares about her In Florida, and it is obvious she does not. Being short with her because he doesn’t want to upset her is pretty lousy of him. He may not know what to say, but he could offer some compassion.

Three weeks is not a very long time for her to become accustomed to her new lifestyle. It’s difficult to accept change when one has all ones mental faculties. Having dementia makes it much, much harder. It’s time to throw her doctor under the bus. Tell her when her doctor says she can go back to Florida, you’ll take her back. You can even say that her husband is remodeling the house for her and it’s not done yet. But make sure you tell her husband about this fib.
You really will make yourself crazy trying to explain the truth to her. Good luck. Come back with updates. We care!