Lucien Monnart was one of three sons of Victor Monnart, a cognac distiller. Knowing that his sons didn't get along, Victor broke down his famed distiller and hid the equipment in three different spots and told each of his three children where one was in hopes they'd work together. After his death though, the sons kept fighting until they found the third piece in Albertus Grotto in 1275, but were attacked by monsters in trying to recover it.
If Geralt says nobody dies today: the three brothers, realizing how stupid their feuding was after their lives were in danger, agree to put aside their differences and work together.
If Geralt tells Hugo he'll deal with the brothers: Auguste and Lucien are killed and Hugo is grateful, having believed they planned to kill him as soon as they got their hands on the last piece.