Heres another Italian...

I palori nimici fannu ridiri, chiddi di l'amici fanni chianciri. Translation: The words of enemies make you laugh, but those of friends make you cry. Meaning: Ignore what your enemies say, buy pay attention to the words of friends. Nun si po' aviri la carni senz' ossu.

And another one...

Don't tell your friend everything that you know; remember that one day you may have him for an enemy.
(Sicilian Proverb)