I think you are in error. She made her case in front of the media - not in a court room. There’s a difference. She accused him and now has to prove her allegations if pressured to do so. The burden of proof is on her - he is the defendant.
I'm not a lawyer, but I've met one.
Cain is in a situation where he may feel that he needs to defend himself from attack, but he is not a "defendant".
"Defendant" is a legal term.
Should Cain resort to the courts by filing a lawsuit claiming damages due to libel or slander, then he would be the "plaintiff". The woman making accusations against Cain would be the defendant, perhaps along with others who have repeated what she said or printed it.
The burden of proof would be on the plaintiff; that is, Cain.
Cain would have to prove, by a preponderance of the evidence (more likely so than not so) that the defendant he is suing made statements, that the statements were defamatory, and that the statements were untrue.
If the jury believes Cain, they can award damages, both compensatory or punitive.
The defendant will be provided an opportunity to contest the claims made by the plaintiff. They may provide evidence that the defendant never made statements at all. Or that the statements made were not defamatory.
Also, the truth is a defense against claims of slander or libel and I believe it is termed an "absolute defense"; that is, nothing further is required. Even if the defendant made the statements, and even if the statements are defamatory, if the statements were true, then there is no libel or slander and there is no obligation on the part of the defendant to pay the plaintiff.
In cases involving public figures, there may be a burden on the plaintiff to prove "malice". I'm not sure what is required to prove "malice", but I believe that making false statements that one KNOWS are false should qualify. If the woman in this case is purposely making false claims, I think that is a malicious act designed to do harm to Cain, not just to furnish information to the public.
I'd be happy to have any lawyers on FR comment if I have gotten this wrong.