Before you decide to pull a funny by changing your friend's birthday after they left their Facebook logged in, take pause. Your change could be there for awhile. I opened my news feed, and found this story among the updates that Facebook felt pertinent for me today:
Ooops. It seems the quote from their help page regarding birthday changes is accurate, but it still makes me ask "Why?" I honestly can't think of a good technical reason for this limitation. Their help pages refer you to their community standards on this matter, which leads me to believe there is no technical reason. Since the community standards makes no explicit mention of a birthday change policy, I'm inclined to believe they just don't want you to do it.
These types of policies make me believe that they are a bit out of touch with their user base. Or, perhaps all the engineers at Facebook are just playing this prank on each other and want it to stick for before it can be undone.
Yeah, I'm going with option 2.