There are genetic conditions, though, that only manifest themselves later in life, so considering that "stupid" is working with the assumption that "hereditary=present from birth." It's actually just a product of ignorance about what CF, specifically is.
My mom has done the "it's your dad's fault" thing, as well. She also denies that children of carriers have a 1/4 chance of having sick kids, arguing it's guaranteed, every time, that they'll have CF (as all 4 of her kids did), and that it's a lie by the healthcare establishment to create more patients by encouraging parents to take the risk. (When carriers father healthy children, or healthy children and sick ones, or sick kids and a single healthy one, it's obviously because the mom was sleeping around. Hah hah.)
Along with all her other ... let's say, "unorthodox" views about genetics, science, and so on, it pisses me off to no end.