Nick stumbles into Kat on a cold night in Paris and picks her pocket, but you can't steal from another thief, silly. So Kat one-ups Nick. Okay, she three- or four-ups him: she steals his wallet, scopes out his I.D., and then breaks into his apartment and waits for him to come home.
When you think about it, this set-up is similar to how Kat met Hale, but in reverse. This time Kat catches Nick, and she recruits him.
Nick's recruitment to Kat's band of thieves—he's a cute stranger, but a stranger nonetheless—is a source of conflict in the last act of the novel. Hale thinks Kat shouldn't be recruiting just any boy off the street, but it's hard to tell if Hale's issue with the dude is a trust issue, or a jealousy issue.
Both Kat and Hale come to realize that Nick really isn't to be trusted. They find out that he's the son of Amelia Bennett, the Interpol agent who has been hot on Kat's dad's tail. But by this point in the book, they're all in the heist too deep to ditch Nick.
Kat does end up leaving Nick high and dry in the Henley after they steal the paintings, though. Nick says that he initially joined the crew to help his mom catch Kat's dad, but he grew to like Kat over time. He says,
I thought you'd help me catch your dad at first […] and then I thought I could catch you—stop a robbery of the Henley be a hero. But... (36.43, 36.47)
Right before Kat leaves him, he claims he did it all "because I like you" (33.65). Do you believe Nick? We kind of do, and we wouldn't be too surprised to see a romance heat up in the sequel.
See, Nick's one of those characters that serves as a kind of perpetual bait-and-switch in this novel. Do we think he's a good guy or a bad guy today? How about now? Now?