The Duchess and the jeweler need each other. She needs his money, and he needs her daughter Diana. They have an unusual bond. The two of them are intimately contrary to one another. They are friendly enemies. One fears the other. They have a strange relationship that involves cheating one another:
They were friends, yet enemies; he was master, she was mistress; each cheated the other, each needed the other, each feared the other, each felt this and knew this every time they touched hands...
One would say that there is a lack of integrity on his part and her part. She is willing to sell an invitation to see her daughter, and he is willing to buy that invitation. Truly, the two have a business relationship. They use one another for each other's personal gain. She knows that he needs her, and he knows that she needs him. He knows her personal weakness, such as her gambling addiction. He is willing to sustain her in her gambling addiction for an opportunity to see Diana. She knows his weakness. She knows that he is writing a check for fake pearls because he desires to see her daughter Diana.
No doubt, the Duchess detests the jeweler's weakness, and he detests hers. Still, they agree to use one another:
...his conversation/negotiation with the Duchess is an exercise in duplicity, deception, and hypocrisy. Under the veneer of good manners and social graces, Bacon and the Duchess are just vicious animals.