This quote is very popular, but the moral of the story you are referencing is the opposite. Geralt rejects this point of view at the end, because inaction is also action.
I think is so popular because of the Killing Monsters trailer.
He does end up actually making a choice. But the choice is not between killing Renfri and Stregobor (which was the initial lesser evil choice) but he chooses to stop Renfris' gang from killing innocent people to draw Stregobor out. Renfri could have then left and look for another opportunity to kill Stregobor. Geralt told her at least two times to leave and she refused saying she made her choice, attacked Geralt and died. So Geralt ended up killing Renfri but as a result of her choice, not his. And he did stop Stregobor from taking her body and study it (which was his initial intent). So he did definitely not take his side. I would argue Geralt chose between good and evil, not between two lesser evils.
He does end up actually making a choice. But the choice is not between killing Renfri and Stregobor (which was the initial lesser evil choice) but he chooses to stop Renfris' gang from killing innocent people to draw Stregobor out. Renfri could have then left and look for another opportunity to kill Stregobor. Geralt told her at least two times to leave and she refused saying she made her choice, attacked Geralt and died. So Geralt ended up killing Renfri but as a result of her choice, not his. And he did stop Stregobor from taking her body and study it (which was his initial intent). So he did definitely not take his side. I would argue Geralt chose between good and evil, not between two lesser evils.