He's not the first, and he won't be the last. Even if you disregard your own time and play against him, and win 9 out of 10 times, it will turn out that you just got lucky. But in the one game where you lose due to a timer, bad morale, or a risky move, it will be claimed that everything happened as expected.
You can't explain or prove anything to such a person; sooner or later, they will learn from experience. Or they will find a group of equally stubborn newcomers and only play with them until they grow up and start playing with stronger opponents.
For now, I would recommend cleaning up all the mess that has been created here and sending this person to take a break for trolling and flaming. I would do it myself, but since there is a moderator in the discussion, I'm just making a suggestion.