Beauty and the Beast
At some stage through Chapter 3 when you go to the Swamp Forest you’ll find Carmen wandering around by the landing. When you talk to her she explains that she’s in love with someone who has lycanthropy and she’s looking for herbs to cure him. Geralt agrees to look for her and tells her to meet him back at the Eager Thighs Brothel.
Make your way to the Druid’s Grove and speak with the Hierophant. He’ll give you a number of options including making a shirt made with Fool’s Parsley, making an alchemical potion or true love.
When you return to Vizima and ask Carmen who her lover is she refuses to tell you. You won’t be able to do anything until you know the identity of her lover. During the quest Lock and Key you’ll figure out that Vincent Meis is the werewolf. If you kill him during that quest then this quest will fail.
The Fool’s Parsley Shirt
Deciding to let Vincent live hunt around the Druid’s Grove or the Golem Cemetery in the Swamp Forest for the Fool’s Parsley. Once you have at least five of them bring them back to Carmen. She’ll weave them into a shirt and tell you she’ll give it to Vincent later.
The Lycanthropy Potion
When you come back to Carmen the next day she’ll say the shirt had no effect on Vincent. You’ll have to try the potion instead. Make your way once more to the Swamp Forest and talk to Kalkstein in the Mage’s Tower. Ask for the potion and then leave for a while and return. Kalkstein will tell you he’s made the potion but it’s still missing one ingredient, Virgin’s Tears.
Make your way back to the Temple Quarter. There are two places where you can find the Virgin’s Tears. Either head to St. Lebioda’s Hospital and find a nurse, a random townswoman in the Temple Quarter or Trade Quarter (you’ll need to ask a few of them before finding someone suitable), or from Siegfried.
Take the completed Lycanthropy Potion back to Carmen who will try it out on Vincent. Unfortunately the potion doesn’t work either.
Speak with Carmen and you’ll discover that she’s unsure whether Vincent really loves her. Head over to Vincent and have a chat with him. Convince him that he doesn’t have a future as a Werewolf and the next time you speak with Carmen she’ll let you know that Vincent is no longer a Werewolf.