What advice does Herbert offer about Magwitch? Why does he say Pip should wait to tell Magwitch that he cannot accept his money?

Herbert passes Pip a note during Magwitch's great confession that changes the logistics of all persons involved. He informs Pip that Arthur (driven to alcoholism by Compeyson) was in fact Miss Havisham's brother, and that Compeyson was indeed the groom who'd left Miss Havisham at the alter. Pip may not want to accept Magwitch's money, but he has to respect and think through what his decisions cause in the broad picture.