This has the potential to be a really fun assignment, so let's brainstorm a few ideas to get started. The first step is to choose an event to write about from Herbert's perspective. One possibility would be to write about Pip's childhood, based on what Herbert has learned from Pip about that time in his life. To do that, we would need to determine how much Pip would tell Herbert. Since Herbert is Pip's best friend, we would assume that he has probably told him quite a bit. Remember, though, that when Pip comes into his fortune, he grows disdainful toward the friends and loved ones of his past and often thinks that he is better than they are.
To write about this event or set of events, one might start like this: “Over time, Pip has told me quite a bit about his childhood. I'm surprised really by how cynical he is about his past and how he looks down upon the people he once knew. There is one incident that really strikes me, though, and that is the time Pip helped that escaped convict.”
Then go on and describe the incident as Herbert would, perhaps expressing special surprise at Pip's agreement to help or at the convict's protection of Pip.
Another possibility is to describe an event that Herbert actually participates in, and a good choice here might be the incident in which Pip and Orlick meet in the marshes, and Herbert and company arrive just in time to save Pip's life. Herbert's narrative might begin something like the following: “I have never understood why Pip went to meet Orlick in that marsh! That meeting almost killed him.”
Then go on to describe Herbert's actions and feelings about his participation in the incident.
Remember that Herbert is generally a friendly, cheerful, optimistic character, so be sure to reflect those characteristics in the tone of his narrative.