Like what, exactly? Negative proofs are famously hard to come by. She says that they were alone, no one else to confirm or deny her story. So, the setup is this: who do you believe? It's the classic he said/she said. No way to affirmatively prove, or disprove.
At best, she tells a bizarre story.
Until this accuser does so, Cain can simply say the same thing: she's obligated to put up or shut up and in the meantime, until she does so, his reply to the media should be, "Don't waste my time and the American people's time with such filth and distraction."
Cain needs to challenge this accuser this way instead of ignoring it or knee-jerk blaming the media.