The ending (spoiler alert!! )

At the ending, the bartender friend tells the young man to sing with the wine to his love, who doesn't love him back, to win her.
Who told the bartender about the wine?

The two villains didn't tell him and at the end they thought it wasn't even true. Who was left to tell him?
It makes no sense, plus the wine didn't work if the intended victim didn't really love the person,it didn't work on Gina.
So why would the bartender think it would work for the young man who said the girl he was after didn't like him or even wanna hear from him??