I told my friends so that they could understand things about me, like why I cough all the time, why I take enzymes and why I have to go to the hospital all the time. Everyone treats me the same, but now they understand why I'm like this, so its all good. I love to freak out my friends with things like my port!
However, some of my friends just don't understand full stop. When I found out about the life expectancy, obviously I was scared, but when my friends asked what was wrong, they thought I was over-reacting, and probably trying to get attention. They refused to listen to the fact that rather than forget about it, I need to be able to accept it to be able to carry on with my life.
So you will get friends who wont be able to deal with the whole hard core CF side, but as long as you don't jam it down their throat, they should be fine. If friends pick on you, treat you weirdly or different because of it, they just don't udnerstand and its clear that you shouldn't waste your energy trying to make them.
Hope everything goes well.