Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on his breast at supper, and said, Lord, which is he that betrayeth thee?
Then (or, 'But') Peter, turning about - showing that he followed immediately as directed.
Seeth the disciple whom Jesus loved following; which also leaned on his breast at [the] supper, and said, Lord, which is he that betrayeth thee? The Evangelist makes these allusions to the special familiarity to which he had been admitted on the most memorable of all occasions, perhaps lovingly to account for Peter's somewhat forward question about him to Jesus; which is the rather probable as it was at Peter's suggestion that he had put the question about the traitor which he here recalls (John 13:24-25).
Peter seeing him saith to Jesus, Lord, and what [shall] this man [do]? - `What of this man?' or, 'How shall it fare with him?'