Here's a wild idea to consider: the "other disciple" that knew the high priest and got Peter entrance to the yard was probably John himself. He often referred to himself in his own book in the third person. Peter being questioned as "also" being a disciple might imply that John was already identified as one, but John was still respected enough to get Peter into the courtyard. If this is the case, Peter's denial is all the more crushing: he stood near another disciple and claimed no connection to Jesus. This is indeed a sad erosion of the core group of followers that Jesus had built.