Once Billy recognized Bart through the flashing lights, he finished off his shake and tossed it in the trash. He walked up and gave a small, two fingered wave to show that he was there, but didn't attempt to talk in order to avoid distracting Bart from the game. He was familiar with how competitive guys their age could be when it came to fighting games, especially in public where one wrong move could easily get you laughed at or heckled.
He was a patient guy, willing to wait until the young speedster was finished.