So I'm thinking of suggesting to my wife that her father is lying about dying.
That's a big assumption to make. Why do you think her father is lying?
When I talked to him, he told me that he was having a disease but he didn't know the name.
While you shouldn't accuse him of lying, it is okay to talk about your concerns with your wife. She should know the truth.
I will tell her.
Just because he didn't know the name of the disease does not mean he is lying about it. It's good to speak up and inform your wife that you think her father is misleading her. However, keep in mind that you may be wrong.