It sounds like your father has a very common fixed false belief (called a delusion) about your mother's infidelity. People with Alzheimer's disease often have these fixed false beliefs stuck
Delusions of infidelity are rarely based in some dark hidden or real past experience. It almost never helps to argue or explain that your mother is too frail, busy, or too moral to be unfaithful. Likewise, this is not the time to remind your father of his past infidelity, even if you are sure he is "projecting his guilt about past transgressions on to your poor mother." Your father is likely afraid of being abandoned by your mother, perhaps at a time he is feeling especially vulnerable or dependent on her.
in their minds, and no amount of rational explanations or arguments will convince them otherwise. Rarely, these delusions of infidelity or delusions of theft (family members' stealing from an older person) occur in the absence of an obvious memory disorder. These rare single delusions may be diagnosed as late-life psychosis.
You can encourage your mom to use a light touch in response: "I'm a one-woman man and you're stuck with me with me for now." Help her understand that denying her infidelity will only frustrate both of them and exacerbate his berating of her. Distraction might work just as well. When he berates her, she could just say, "well, we have plenty of time for that discussion, but you look like you need your favorite show, a walk or some ice cream." Asking him to help her with some task may distract him as well. Help her understand that these distortions in thinking are intruding on him. He is not choosing to make her life miserable.
She will also need your help in finding some time for herself, away from his constant accusations. That kind of stress will take a toll on her patience and her health. If none of these communication strategies work, and the accusations escalate, he should be seen by a physician (ideally someone familiar with use of antipsychotic medications in older people). There are risks, benefits and trade-offs in use of these medications, but if it is important for them to stay together at home, it may be necessary.
One note of caution. Ignoring escalating delusions is dangerous. Often delusions do require medical evaluation and treatment to break the cycle or the repetitive berating. It's just best to try non-drug strategies first.