We have nailed down the precise quote (“Whenever a friend succeeds, a little
something in me dies”) to Gore Vidal. It is, however, only one
manifestation of a cast of mind that goes back at least as far as
Rochefoucauld, who said: “There is
in the misfortunes of our friends something not entirely unpleasing.”
Jonathan Swift drew this out into a longish poem. I bet there is an apt Florence King quote somewhere, too.