Minor point compared to the rest of it, but since this 'somebody' is relying on it as a supporting factor, you might want to tell them that the Theory of Relativity says nothing of the kind - whether they're invoking Galileo's relativity principle or Einstein's Special and General Relativity. It's a childish attempt to impress with scientific authority, which ought to give some clue as to the reliability of the rest of what they claimed. Not to mention the gullibility level of the person who said it and their opinion of yours.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'