@RevereShin
He was right to assume the match was over, had he been using something other than a training blade for a casual duel (most people probably wouldn't use their best weapon for a casual duel), it would've cut her head off, his swing, if extended to where his blade would've been unbroken, passed through her neck.
He'd won, spared his opponent's life, paused in a position (his blade against her neck) where he could've easily killed her without her being able to even maim him in return (AKA a position of complete advantage), backed off, and that is proper form for winning a duel without drawing blood.
She then sneak attacked after the fact, and the ref, who'd wanted to end it when she prepared the attack (which he broke) because she used force beyond that of a casual duel, called it in favor of the one who broke the rules twice instead of the one who'd rightfully won.
It's like if someone in a professional wrestling match was pinned for several seconds (beyond the two needed for it to count, then the person pinning him got up, and the pinned guy was declared the winner after then knocking him out with a blow to the back of the head using a set of brass knuckles he'd slipped into the ring. He'd lost, match was over, and he still attacked, and did so with illegal force.