How does Borden and Olive's relationship change in Section 4, Part 2?

Although Olive is hired as Borden's assistant for his magic show, she ends up moving in to his rented flat and becoming his mistress. Soon after, she states she only wants to be his mistress - not his assistant - and admits that Angier originally sent her to be a spy and obtain Borden's secrets. She insists she's told Angier nothing and wants to be with Borden. Borden sends her back to Angier with a message.