Nick Wilde: It's called a hustle, sweetheart.
Chief Bogo: Life isn't some cartoon musical where you sing a little song and all your insipid dreams magically come true. So let it go.
Judy Hopps: What did you do that made Mr. Big so mad at you?
Nick Wilde: I, um... I may have sold him a very expensive wool rug that was made from the fur of a skunk...'s butt.
Judy Hopps: Oh, sweet cheese and crackers.
Nick Wilde: Everyone comes to Zootopia, thinking they could be anything they want. But you can't. You can only be what you are. Sly fox. Dumb bunny. And that is not wet cement.
Judy Hopps: Life's a little bit messy. We all make mistakes. No matter what type of animal you are, change starts with you.
Chosen answer: Finnick didn't say anything that could be used against him and she had no proof that he was delinquent on his taxes, as he said, "She hustled you good!"
Finnick was involved in all of Nick's scams so shouldn't he have been charged as an accessory and therefore also forced to help Judy?
Nick's crime was not paying taxes. Judy had no evidence that Finnick was guilty in any tax evasion or that he even participated in earlier scams.
Judy witnessed Nick and Finnick involved in a scam involving popsicle sticks that he and Finnick used to create pawpsicles and sell them to lemmings. They also sold the popsicle sticks to mice for construction. The scam involved melting down a Jumbo-pop to make smaller pawpsicles and then reselling them.
Which, as Nick explained, was perfectly legal. Also, Judy had no desire to arrest Nick, only to force him to help her. His partner was of no use to her, so she let him go.