Negative and Infinite ATK in Yugioh
1)  No, negative numbers are not possible on monsters.  It is true that this is not explicitly stated in any of the official rulebooks.  However, it is most common that such values in games refer to non-negative integers, and therefore it can be assumed that it is the case unless otherwise stated.  It is the responsibility of the rulebook to explicitly state if negatives were allowed, not the other way around.

2)  No.  There is noting even remotely true about the statement, to the point of which the very concept seems rather random.  It's possible that such a glitch might exist in one of the video games, and that your friend might have been referring to that.  Even so, that is an unintentional (and theoretical) computer glitch, and does not reflect on the real game, which is not limited by such issues.