I (42m) found out that my father(80m) is cheating on my mother(82F). How do I tell her? Or maybe I shouldn’t?

This is an odd question but here it goes. My parents have been married for 62 years. They are in their golden years and are not getting younger. My father and mother live in a comfortable house (owned and paid for) and do not lack for anything. My father decided to buy an apartment and my mom thinks that it’s for my sister and I and our children to come visit them. In reality it’s the other way around. He bought an apartment for him and his “gf”(58) to be close to my mother. My mom knows nothing of his “friendship” with the woman. <br><br>I confront him yesterday and he told me that it’s none of my business and that I’m to tell my mother the truth that he bought the apartment for all of us. I’m in a dilemma. I don’t want to hurt my mother or tell her something that might hasten her death. But at the same time I don’t want to lie for my father. Redditors, what do I do?