“No one is completely unhappy at the failure of his best friend.”
From his book Groucho and Me. It is a variation of a maxim by 17th-century French nobleman François de La Rochefoucauld: "In the adversity of our best friends, we often find something that is not displeasing." (Maxim 99 from Reflections; or Sentences and Moral Maxims, 1665 edition.)
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Groucho Marx 117
American comedian 1890–1977Related quotes

“The only real failure in life is not to be true to the best one knows.”

“My best friend is one who brings out the best in me”
Actually due to Harris Weinstock: "My best friend is the man who can bring out of me my best, and your best friend is the one who tends to bring out the best in you" (May 1914) Attributed to Henry Ford as early as 1948.
Misattributed
“Is there, friend,' he cries, 'a spot
That knows not Troy's unhappy lot?”
Source: Translations, The Aeneid of Virgil (1866), Book I, p. 23

“An extrovert imagines that the people around him are his best friends.”