In Chapter 58, Pumblechook is trying to take credit for Pip's previous good fortune -- for the fact that Pip got to go off and be educated. Because of this, he tells Pip to tell Joe that he (Pip) has seen the man who had helped him (Pip) out. He is supposed to imply to Joe that Pumblechook is the one who is responsible for Pip's "great expectations." You can see this in the following quote spoken by Pumblechook to Pip:
"I will tell you, young man, what to say to Joseph. Says you, "Joseph, I have this day seen my earliest benefactor and the founder of my fortun's. I will name no names, Joseph, but so they are pleased to call him up town, and I have seen that man."
Pip refuses to do this. When that happens, Pumblechook starts to make him look bad in front of all the people. He makes it look like he is the benefactor and that Pip is an ungrateful person.