My apologies, but this is an adult pulmonary physician and your question is best answered by a pediatric specialist
CF is a genetic disease so both you and the father must carry the trait. If you tested negative there is no possible way he can have CF. if you are really concerned you can ask for another test to be sure you are not a carrier.
I know this is a bit late, but the newborn screening only tests for about 56 of over 1500 known CF mutations, as does the carrier screening. We have had both. It is by all means possible her and her child carry one of the other 1450 mutations.