Can ground work ALWAYS beat standup fighting.....????

My friend enrolled in BJJ at the same time when I started Muay Thai. He claims that BJJ or any type of ground work can beat any standup Martial Art out there since once the striker is on the ground then he won't know what to do and he backed it up by showing me a few vids of Royce Gracie demonstrating his BJJ with standup fighters and defeating them. Is this really true though?. Can any type of ground work beat a striking art in and outside of the ring everytime?.