Since cystic fibrosis is recessive and both parents are not showing it despite the fact that their child was affected, it should be deduced that both parents are heterozygous for the gene. This gave a 25% (or 0.25) chance of having an affected child. And since there are only 2 choices for the sex, probability will be 50%.
Cystic fibrosis is an autosomal recessive disorder that affects lung function in humans. If a couple, who are both unaffected, have an affected child, what is the probability that their next child will be an affected girl?