I'm going to play devils advocate here and say that I think they did what they thought was best for the other passengers on the plane. They didn't know what her issue was, maybe she was pasty white and sickly looking in addition to the coughing and trouble breathing. It's in the best interest of the pilot, who's ultimately responsible for the flight, to make sure that SHE is taken care of, as well as what is in the best interest of who is left on the plane.
With all the things that are available to "catch" nowdays, you have to do what's best for the majority, not the minority. What happened if it was more than just a cold and she coded on the plane, AFTER she had all those problems. Who's A%s would be in hotwater then? Not her's, the pilot's.
To cover your butt, if you are worried about something like this; take a note from your doctor about your CF and a little informational print out about CF next time you take a trip.