I'm sure it's really scary having a friend go through that (it's scary for us to go through it too!), but you should know a lot of people with CF cough up blood sometimes. I myself do it on a semi-regular basis. How serious it is depends on a few things, like how much it is (I usually bring up a tablespoon or 2) and how frequently it happens. A lot of blood can be serious, and if it's happening very frequently it may indicate it's time for a "tune-up" in the hospital (when I need a tune up I can have small bleeds almost every day), but it really varies depending on what's normal or not for the person.
If your friend didn't seem too startled by it I would say you shouldn't worry too much. If she's never done that before she should definitely let her doctor know (even if she does it regularly it's important to let her doctor know how frequently and how much blood is normal for her and if anything changes). I would say DON'T make a big deal about it, I don't find it very helpful when people around me start freaking, because <i>I'm</i> scared when I bleed and if other people get all upset it makes me less calm, and also if other people start freaking it kinda makes me feel like a freak! I would just say to let her know you're there to help her and comfort her and let her vent if she needs to... I know sometimes I put so much effort into making things out not to be a big deal so my friends won't get scared, that I end up being scared by myself because my friends don't even know there's anything wrong. Which is a shame, because thank Gd I have some really great friends who would totally be there for me in those times if I would just tell them.
That's my take on the situation, it could be different for other people though.