Dad dating ex-mother-in-law!!?

Sorry but I agree...he's a grown adult and you wouldn't like him telling you who you can date now would you? It's his life he can date who he wants, you don't have to like it but you have no say in the matter. He's lying to you because he knows you won't be understanding and he doesn't want to deal with confrontation in his older age, its time for him to have fun....try telling him you don't agree with it but you want him to be happy and if this woman makes HIM happy then so be it, that's what is most important here. Would you rather he be lonely and miserable just to suit what you want? That's not very nice of you.