Richard,
I know it's terrifying to be on that side of the conversation. I've sort of been there...but I was the potential girlfriend at the time. I swear, I didn't sleep for three days!
My best advice, though, is to become as educated as you can, but don't get hung up on the statistics. They are averages that include people with CF who died in infancy to people who died quite late in life, and so it doesn't really give a good indication of how long your friend might live. There's no easy way to get over the sadness and worry, but as there's nothing you can do about it, try not to dwell on it.