During the final confrontation with Letho, he talked about two other witchers, which may or may not be alive and on the Path. But he never mentioned this assassin in particular.
In the prologue, Geralt can talk about him with Triss (autopsy results) and a Temerian Recruit (from where he came), the soldier is near Crinfrid Reavers. Before entering the battle tower, after conversation with Shilard Fitz-Oesterlen, Foltest asks Geralt about him.
Later, in Epilogue, when your memory recovers, you see the King of The Wild Hunt, Yennefer, Geralt, Letho and three witchers (Serrit, Auckes and maybe this assassin).