First thing stop judging your father! He needs your help, not your anger.
My wife had Alzheimer’s disease. Symptoms started at age 54, she was dead by age 63. You said that you had not heard of ‘early onset ALZ’, this tells me that you have a lot of study to do related to Alzheimer’s. ALZ is unlike any other terminal disease (and YES it is 100% fatal) in the things it does to REALTIONSHIPS.
As a child of this relationship you cannot imagine what it is like to have your spouse walk away from your marriage. Your mother’s disease has caused her to stop participating in her marriage responsibilities. Her disease has caused her to be unable to uphold her role in this marriage. And this has been happening longer that you realize. By the time symptoms are recognized by others, the person with ALZ has been having problems for 4-5 years. Your mother has not been a wife to your father for some time.
You made a statement that “My mom calls my dad her husband daddy she knows he is her husband.” This is not necessarily true. My wife had NO IDEA who I was. I had long ceased to be her husband. At best I was a friendly person who helped her feed and dress every day. I was no longer her husband and (difficult for me to accept) she was no longer “my wife”.
The decision to seek affection and (as yucky as it is to you) sexual pleasure outside of a marriage that has been destroyed by Alzheimer’s is a deeply troubling and personal choice that the non-ALZ spouse has to make. Do not judge your father too harshly, he has had his world torn apart even more than yours.
My suggestion is to talk to him. Acknowledge that you understand why he would seek the love that his wife/lover can no longer give him, but you do also need to see to mom’s care. Your goal is NOT to try to patch up a marriage that has ended due to Alzheimer’s (and YES Alzheimers does end marraiges) but to tell dad that he at least needs to insure that financially her needs are met. If there is a forced divorce this can really mess things up in financing mom’s inevitable burden of care.
Please search out “Jan’s Story” on the internet. It is the story of a CBS news reporter who sought love away from his ALZ affected wife. (There are books and videos) I think it may make the situation clearer for you. The husband stayed married to the ALZ spouse and saw to her care needs, BUT he also met his own needs in another partner who fully understood the situation and even visited the wife in her facility.