Quote Originally Posted by DE
So HOW exactly would he dodge a sniper shot?

Kirito dodges shots either by the beam, or by his targets eyes. The first shot of a sniper gives the target neither of those things, so dodging should be impossible.
Sinon came to the conclusion that DG already saw her, and thus her position was compromised. A shot fired from a compromised sniper, first or not, will be preceded by an assist-line. If they were visible to all, then Sinon would not have come to that conclusion. She very specifically said it was a system assist (aka assist-lines).