Out of the abridged edition that I have, I found the passage where Biddy asks Pip why he wants to be a gentleman.
"Do you want to be a gentleman to spite her or to win her admiration? Because if it's to spite her, I should think you could do that better by not caring about what she said. And if it's to win her admiration, I should think it wouldn't be worth winning."
Biddy think it's ridiculous that Pip would want to change for such a heartless girl. She realizes that Pip developed a crush and he can never get away from it. However, there is nothing she can do but remain the "teacher" as she used to be in Pip's life by pointing out his motives. He is only speaking and acting upon his emotions. He hasn't thought any of this through, so she is serving as his logical side.