Were Sol and That Man childhood friends? Going by That Man's short story and the flashbacks in Xrd, it seems like Frederick and the Gear Maker have know each other longer than either of them have known Aria(who they both met in college).
You could argue that everything started with the Dawn of Revival.
Humanity had just barely managed to piece it’s own social structure back together. So with a group of ambitious scientists, understanding everything that had happened in the past was key to unlocking stability and prosperity in the future.
The “Apostles” who were followers of the Original who predicted the events that transpired in 1999 were the ones who helped rebuild humanity from the ground up… though there were dissenters, most of humanity embraced Magical Science and Magic Energy as a natural resource, even accepting byproducts like Zeal.
It was these people who presented mankind with the ‘knowledge’ on how Magic could be studied and worked with in order to further preserve their prosperity, even though the Sanctus Populi knew so much, there was much they still didn’t know… especially about the Backyard itself… as even the Original forbade anyone from touching that ‘sacred’ place.
Men like Vince Macdonnell had a vision for using Magic to forge a brighter future… so he called together experts in the field… people who had been studying Magic as a resource for the past 14 years or so: Frederick of course, Aria… and “That Man”. Their goal was a humane one, and at the same time ambitious in every way: find a cure to all diseases and further Mankind’s Evolution.
So because of their growing knowledge, the “Ecosystem Evolution Project” was set in motion.
In a way, there’s a certain point in which humans reach their “limit” in development and begin to idle and stagnate, only to drift off in to old age and oblivion. It’s very rare for evolution to be “seen” from one generation to the next, as in the natural world things would be moving along much more slowly (like comparing a lizard to a fish… if there are any species that exist “in between” as a hybrid of the two, they are hard to spot except in fossils).
So because of this, they felt that Science was a means to 'speeding up the natural process’.
How far back Frederick had known his peers is not yet known… but GG history tells that had he not chosen to work with those peers, he would have had other work in store for himself.
For now, we can only assume they were working with each other long enough that what they had in terms of friendship was something special, and not something easily forgotten.