You can not install the belt wrong onto the water pump to the best of my knowledge, and i've changed a few timing belt and water pumps. He may not have tensioned the belt right, but i doubt that would have anything to do with that. Possibly a defected new water pump? If the water pump itself is not leaking where he put it on, then it's not his fault. From reading your post, it sounds like you need a new radiator cap.