Here's how I looked at it when my ex husband and I were married, we made sure we were "set up" (life insurance, disability insurance...) for when something happened to him. WE both really wanted children and making the decision wasn't an easy one for me, but my biggest "factor" was that when he's gone someday, I wanted a part of him left behind. And that little part of him would be our child.
My ex and I aren't together anymore, but for reasons not having anything to do with CF. And I would still NOT do anything different! He is an amazing father, and I LOVE being a mother.
The truth of the matter is (and I know many CFers might disagree) is that NONE of us are guaranteed tomorrow. At least with CF, or some other "terminal" disease, people know that they have a shortened lifespan and I think that might allow them to fully savor all the wonderful things in their life, including children. I could get hit by a bus tomorrow. If they both want children, I don't think CF is a good enough reason not to.
I've got lots of info on my website about IVF, ICSI and the sperm retrieval, and I'm happy to answer any questions your son and his gf might have if they decide this is the right route for them. cysticfibrosismaleinfertility.com