How does Proteus betray his friend Valentine in Two Gentlemen of Verona?

In Two Gentlemen of Verona, Proteus arrives and falls in love with Silvia, Valentine's beloved. He forgets his love for Julia and his friendship with Valentine. Valentine tells Proteus that he and Silvia plan to flee to Mantua. Proteus proves to be a faithless friend and discloses Valentine's plan to the duke, her father.