I partially agree with Deb. While you might not be able to escape living with CF and all the treatments, you certainly don't need to let CF define you. Whether or not the person mentioned in the OP understands the struggles of a person with CF we can't know. What if that person has gone through a particularly tough time themselves and is able to relate? Does one need to have a particular set of problems to identify if another person is letting CF define their life?
I see quite a lot of people fall into this trap and it's not just people with chronic diseases. Some people fall into a role and portray that role ad nauseum. Sometimes that role can generically be "the victim," as an acquaintance of mine often plays. I brought it to her attention and at first it was met with some fierce opposition, but I think she's starting to see the pattern and the effect it's having on her social life.
Another important thing to remember is that almost everybody is up to their eyeballs in life's crap. Most people really don't want to have to deal with your crap in addition to theirs. Just because they don't complain about stuff they are going through doesn't mean it's not there. Don't assume that of your friends and acquaintances you were dealt the worst hand. Remember that this is a very subjective thing. What may seem like nothing to you might be the worst thing that person has ever been though.
You should be prepared for people who see you dealing with a disease and assume you have the strength and patience of 3 people. Consequently, they will come to you and unload all their problems, which can really drag you down if you're having a hard time of things yourself.
Did you agree with that last paragraph? Good, now let's reverse the roles. Are you dragging people down by (perhaps unknowingly) unloading your problems on your friends, who may or may not have significant problems of their own to deal with?
I interpret the OP's story like this: The OP's friend feels that every time the OP interacts with them, there's something brought up about CF (treatments, being sick, clinic visits, etc.). Because of that, the friend thinks that the OP has allowed CF to take over and define her. I really can't imagine a scenario that would lead to that conclusion on the part of the friend, other than the one I just mentioned. Perhaps this friend is irritated because they are dealing with problems of their own and use social interactions to excape those problems. Only, in this case she's only met by hearing stuff about CF. In this case she's escaping her situation only to be handed part of OP's situation, completely screwing her attempt at escape. Maybe the friend isn't needing to escape anything, maybe they just want to have a nice time with a friend. Again, that plan is screwed when they have to hear about the struggles of CF. I obviously could be wrong, but this is the scenario that makes sense in my mind.