Best bet, a person who left the Graybeards, but would not cooperate with the Blades. She is a pure scholar and has spent her life studying the Dragon Lore. The ultimate goal of her research has always been thwarted, which is to see the effects of the ancient words of power. Your character comes along, the first in eons to be able to use the power and she gives hints to the locations so that you can learn the Shout and demonstrate it's effects to her. She then documents the effects for a book that she will write. This is why I always try to use a new Shout after getting it.
If TES6 takes place in the years following this game, I hope to see that book as loot somewhere.
That's an interesting way to see it. Your theory adds more mystique to it all.
But I can't agree with waiting 4-6 years just to read it in an in-game, TES VI book. Without deraling this into a separate topic, I think a DLC involving your version of the "mystery friend" with a nice 20+ hour chunk of content behind your idea would be more like it. Otherwise we might sit back and ask why something like this wasn't implemented as added content when, at the time, we had the setting to do so.
But until then, I guess all we can do is speculate on who this "friend" truly is.