I have an uncle-in-law (is that a thing?) that exclusively drinks coffee and Diet Coke. I thought he was joking until we went on a 10 day family vacation, and I

I have an uncle-in-law (is that a thing?) that exclusively drinks coffee and Diet Coke. I thought he was joking until we went on a 10 day family vacation, and I never once him drink anything else. Does not compute indeed.