Fred had gone to get another drink, leaving Helena momentarily. He was very protective of her, and didn't like leaving her alone unless it was someone he knew would take care of her. He was about to make his way back over to her when Sirius came in and a grin spread across his face.
He held open his arms as he yelled a big "SURPRISE!" then bust out laughing. He went over and patted Sirius on the back, "Happy Birthday, old man," he said with a wink. He pressed the pint he had got into Sirius hand, thoroughly hoping he enjoyed the party.