Robbing a building requires, at a bare minimum, finding a way inside it. The mysterious man who convinced Geralt to take part in the heist determined the only way to break into the carefully guarded auction house was via an adjacent tower. Geralt thus needed to hire someone capable of climbing up its glass-smooth outer wall and dropping a line down to the others. Only two maestros of the burglar's trade could perform such a feat: Hugo Hoff, affectionately known as "Beef Patty," and Eveline Gallo, alias "the Ermine."
In the end, Geralt recruited Eveline Gallo to join the heist. This was not a very difficult task -- he had the impression the performer was just waiting for an excuse to escape from the circus tent for a moment and once again savor the taste of a true adventure.