At the highest point of a vertical throw, is velocity zero but acceleration still 9.8?

Your friend is right. At the highest point the velocity is indeed momentarily zero, because the ball has stopped rising and is about to come back down. But the acceleration is not zero. Gravity acts on the ball continuously throughout the entire flight, pulling it downward at about 9.8 m/s² the whole time, including the instant at the top. If the acceleration were zero at the top, the velocity would stay zero and the ball would hang in the air forever, which clearly does not happen. The zero velocity at the top is just the turning instant where the upward motion changes to downward motion, while the downward acceleration keeps acting steadily. So at the top: velocity = 0, acceleration = 9.8 m/s² downward.