My intuition is that being able to quantify and visualize 'quality-of-life' automatically, will help support diet and lifestyle changes.
What kind of features/UI would you like to see in such a tool? Would it be better is this was generic (e.g. like Facebook), or something specifically for patients with CF?
The CF market per se is pretty small. I wouldn't look to customize it specifically for them. I also think that "good practices" generalize. If you can come up with some parameters that work well with vulnerable populations, it should work regardless of their specific issue.
Features that might be nice: A text message "alert" when early indicators are noticed and perhaps a means for the customer to put in custom "prompts" so that, whatever their medical condition, they can self-remind (for example: that they might have forgotten to take a particular medication). People with medical conditions often become "forgetful" as their condition starts heading into a downward spiral and the sicker they get, the harder it gets to think about what they most need to do to stop the downward spiral. A reminder to do the right things and an alert that their patterns suggest they might be in need of extra care could well be the stitch in time that saves nine.