Lysander
Tonight was supposed to be the night he would win her back. He would rescue her from the boys trying to take advantage of her. He'd stop her from drinking too much or getting into a fight. And then he'd whisk her away to the dance floor where they would dance and she would remember why she used to go out with him.
And then he sees her sitting on the ground next to Scorpius Malfoy, who takes her cup out of her hands and empties its contents onto the grass. Then he holds her hand.