Help with Mom's behavior

Hi, I am helping my mother to take care of my father who has parkinsons and alzheimers. My father, like many people with alzehimers, needs people to be patient with him, talk slowly and give him some extra time to understand what people are saying. My mother, who loves him, has very little patience with him. She talks very quickly to him. When he does not react quickly enough she raises her voice and treats him like an errant child. She is also saying things in front of him like, "he dosen't understand that...", or "he can't do that anymore." My father gets quite frustrated by their interactions. Most of the time he just takes it, but occasionally he lashes out and also gets nasty. This results in her complaining that he does not appreciate all that she is doing for him. I have tried to talk to her about the need to slow down when speaking to my father and pointing out to her that my father understands a lot more then she then she thinks. All is falling on deaf ears. She complains that I do not have to "put up with him," like she does so I do not understand. I know my mother loves my father and wants to do what is best for him. It is also true that she is assuming the heaviest burden for his care. It just seems to me that things would be easier for both of them if my mother changed how she interacted with my father. Unfortunately she is 84 and I really can't see her changing. I am at wits end on how to deal with this situation. She complains to me that my father is being nasty to her or that she is having problems getting him to do things. I will then step in and not have any problems, just by going slow and being nice to him. Unfortunately, I can't be there all the time. I have a job and do not live that close to them. Does anyone have any suggestions on how I might get my mother to see the light or is this just hopeless? Thanks in advance for any and all advice.