I have CF and grew up with soft, smelly poop. My digestive system is just not as good as my lungs are. I've had a lot of problems with soft poop especially after taking a course of antibiotics. It depends on what she eats though, although whatever it is, she should be taking her enzymes before she eats (they take about 10-15 minutes to kick in!!) I know when i eat particularly fatty, greasy foods, a belly ache with soft smelly soft-serve ice cream poop is to follow. Also,with the fatty greasy foods, it's really common to see oily residue. Just fat that she was unable to absorb.
I take protonix, which is along the same lines as zantac just higher dosage. It seems to help. But, what I swear by is Dannables Immunity yogurt drinks. They taste good, they're easy to eat, and they are truly amazing. I started drinking one a day and about after a week saw great results. normal, regular poop. I didn't have as much stomach pain or gas!! Truly worth the money. They have a lot of different varieties of probiotic yogurts out there, but I found these to be the best results.
I also fall back on benefiber. It's tasteless, (except in water or really weak drinks) mixes great into cereal milk, mashed potatoes, really anything soft that can be mixed up. I find that increasing my fiber helped with my stomach issues and creating more of a normal BM schedule. =) Oh the woes of CF. Good luck to you!!
--Beth