Sean Friedrich is first seen speaking on Veronica Mars when he is shown as one of the players at the poker game at Logan Echolls' house in An Echolls Family Christmas. He was the one that took the money, resulting in various amounts of items disappearing from the homes of each player that took part in that day. He appears again when Veronica interrogates him about Shelly Pomoroy's party. And later again when Weevil also interrogates him on who his supplier is.
When Veronica hears of the news of Carrie's death, she finds newspaper articles and hears reports that Sean had been having an affair with Carrie just before the start of the whole movie.
He attends the Neptune High Reunion 9 years later. Logan approaches him and is silently being threatened.