Doctor No name wasn't there at the beginning of murders. Not sure when he actually started but I don't think they formed a friendship until baby N.
Letby manipulated him but I don't believe he was the motivation for any of the attacks. I don't think he behaved with malice and think he was just a tool to Letby.
He gave evidence about Baby L (the second insulin poisoning), but his evidence only lasted a few minutes. I think this was the first time he was involved in the trial, because it was then that she became upset and tried to leave the dock when she realised who it was.
He basically just confirmed that Baby L’s blood sugar levels were lower than he would have wanted them to be. I’m guessing he was only indirectly involved with this baby (perhaps he was on duty but not directly involved in baby’s care) and they called him due to paediatric diabetes being his speciality (and to freak Letby out).
I imagine it was around then, or perhaps a little before, that they started developing a friendship, which then grew into what it eventually became.