This is proved by the doorkeeper's question to Peter in v. 17: "Are you also one of this Man's disciples?" In the words "also you," she clearly refers to John, already in the courtyard, as Chrysostom indicates.
Peter probably did not dare enter the enclosure of the high priest's courtyard because he was known to no one in the house. Perhaps timidity had also overcome him, though his heart drew him to follow the Lord.
Seeing that Peter remained at the doors, John used the fact that he was known in the house and instructed the doorkeeper to bring Peter in.
Apparently, without any malicious thought or reproach, as Chrysostom notes, the doorkeeper asks Peter whether he is a disciple of "this Man." She adds no reproachful words.
She only lets it be understood that she knows John is His disciple. Timidity overcomes Peter, and he pronounces his first denial of Him whom he undoubtedly loved more than his own life.
Compare the accounts of the other Evangelists: Mt 26:69; Mk 14:60; Lk 22:56 and following. On Peter's love, see Jn 21:15-18.
But Peter needed a lesson in humility. Compare Jn 13:37-38. The Lord permits grace to depart from him, without which every man is powerless.
Apparently these events take place in the courtyard of the high priest's house, where not only Caiaphas lived, but also his father-in-law Annas, though in different buildings and not in one courtyard. Westcott in Speaker's Bible, note on v. 15.