cahn

First class word, Detective! I say Ernst was with Dad in St. Peterburg. Whether he should in Silesia: otoh, Heinrich was at age 16, so why not, otoh, Heinrich is Heinrich the brat, third brother, whereas Ernst is Oldest Son Of Beloved Suhm, that's why not. Also if Ernst was in Silesia, Georgii could have been outed by him directly (to Fritz). So it's more convenient if he's not, and we have roughly this series of events:

Fredersdorf: does thing that would have been okay with Crown Prince Fritz but not with King Fritz
Fritz: discovers Georgii, is told Georgii was personally recced by Suhm, chats with him; Georgii can't resist going from "yeah, he recced me" to "yeah, we totally were bffs those last months in St. Petersburg, he saw a young guy who needed help, he helped, you know how he was"
Fritz: promotes Georgii to batman and makes pointed gesture of temporarily removing Fredersdorf's unlimited access, possibly even sending him back to Berlin to look after things there (this will later gossip-wise, transform in "Fredersdorf fell out of favour/was put back into his old regiment)
Fredersdorf: checks on Suhm's family, which he'd have done anyway, but is also now for the first time since their initial getting together somewhat worried about his place with Fritz, plus he's distrustful of this Georgii fellow but can't tell whether that's instinct or jealousy, so he asks Ernst whether he knew Georgii
Ernst: Hasn't, but of course it's theoretically possible that his father could have had a protegé without him knowing
Fredersdorf: has Ernst look at original rec letter, which is in proper secretary's handwriting, decides the secretary is key, finds out secretary is now in Dresden, has spy interlude in Dresden, returns to the front several weeks later to give Georgii an ultimatum
Georgii: By now has gotten to know Fritz well enough to conclude he's toast if Fredersdorf does spill the beans = > suicide