Hi, I hope this helps. I have missed plenty of school and it is hard has always been difficult for me at first to tell my professors about my CF or now my rejection.
Like laulau said though, The Office of Students with Dissabilities has been very helpful for me, just when you need that extra support. They're always great at breaking the ice too.
Anyways, my advice to you is to just tell her what "end stage" CF is. They are people just you us. Yes, they may be intimidating at times but really I've come to learn a lot about my professors by talking to them about my CF and transplant. Either schedule and appt or go in during office hours. Maybe a letter from you doctor explaining what "end stage" CF is. That might be helpful as well.
I hope this helps a little.
-Anna