Witcher Wannabe is a Side Quest in Velen, with suggested level 10. This is an easy quest, and involves killing a single Ghoul.

Follow the map marker, and you'll encounter the Ghoul at the graveyard. After killing it, you will notice the other "Witcher" scared and hiding.

Talk to the him, and shortly afterwards the villagers will arrive. You have multiple options on how to deal with him.

Tell them he's a fake: He'll be hanged by the village elder if you tell the villages to take him there. Alternately, you can convince the villagers to let him go.

Tell them he's not a fake: Chose to have him work off a debt by working for the village elder for a year. Geralt will also instruct the charlatan to "abandon his trade" by burying the medallion in the earth. Your other option is to tell him to never return "to the site of his failure."