I'm going to assume you mean getting added to his/her policy provided from his/her employer.
If you currently have health insurance now there should be no problems -- part of what HIPPA did was help in that respect -- the last "P" stands for portabilty -- meaning you can transfer your insurance coverage within a 60 day period, you can change companies without any pre-existing conditions clauses coming into effect -- so if you have coverage now and are changing to the other, or you have had coverage up until 59 days prior to getting added to the other persons policy, you are in without a problem.
If you have had no coverage within 59 days of getting added to the new company/policy chance are you might be asked to wait out the waiting period. I believe the maximum legal waiting period is 18 months so it cannot exceed that and I think most companies only go 12 months.
To my knowledge they cannot deny you coverage, but they can have that exclusion for specific needs of CF for the waiting period, so you will have coverage for any other issues and eventually they will start covering the CF stuff too, most likely 12 months later.
Good luck