One of the meanings of "calling" something is to make a correct guess or prediction. When someone correctly guesses a future event, you say "You called it!"
In the example above, the speaker's friend correctly guessed who the winner of the singing competition would be. So she "called" it.
This is an expression that's used in casual English. In more formal English, you can say that someone "predicted" something instead:
She accurately predicted who would win.