As I've heard the most standard break out is:
Mild - PFT's 70% and above - even if you dip down lower, if you bounce back up with treatment you're still considered mild.
Moderate - PFT's 50's - 60's percentile
Severe - PFT's below 40%
Sorry to hear about your friend, hopefully it's only a temporary setback and not permanent. But there is always hope. I am at the severe level, but still live a pretty normal life.