"As soon as"? Even before she bothers to let the guy know?
A hormone-charged boy is akin to a car rolling downhill. Apply the brakes for a few seconds, and it will stop; if persistent application of the brakes does not stop the car, one may rightly conclude something's wrong with it. But tapping the brakes lightly for half a second, waiting awhile (with the brakes not pressed), then pressing them lightly for another half-second, and then waiting awhile more (with the brakes again released) isn't apt to stop any car I know of.
Since I know of no claims of improper legal procedure in this case, the only basis for appeal would have been "Even if everything the prosecution alleges is true, the allegations do not support the current charge." If the girl had claimed to have done anything to encourage the boy to stop beyond making her two utterances, those claims would have been presumed factual in this appeal. Since they weren't, she didn't.
Unless the facts of the case have not been presented accurately in the thread, I would say the California Supreme Court has made a terribly unjust decision.