What did they give her for this?
When my son (no cf) had a bad bout of croup in the last few years, he was given a course of oral steroids which helped immensely. Emily is prone to the 'barking seal' kind of cough seen in croup when she gets really sick as it is; and earlier this year got sick with parainfluenza, the same virus that also causes croup in young children, and she got really sick. She needed a high dose of Prednisone as well as her inhaled steroid 4x/day (with her albuterol & vest every 4hrs) to keep it under control. She was still coughing so hard she was vomiting.
From our experience I would expect a cough that sounds like a barking seal or sounds 'tight'; she may complain of pain in her chest or some trouble breathing. Give her nebs as often as her dr allows it. It's a virus, but keep an eye on her over the next several days to look out for secondary infection... did she see her ped or was it her cf dr that dx'ed the croup?