It isn't proving a negative. Either she worked there when she was 14/15 or she didn't. There should be records/witnesses that can confirm or deny that. A woman who did work there at the time said that no one less than 16 worked there and she never saw Moore there. Either Moore signed the yearbook or he didn't. Experts should be able to prove that with some certainty. Like I've said repeatedly, this is beyond she said/he said. Parts can be confirmed or exposed as a lie. It only takes one of her claims to be proven a lie and it all falls apart.
Even if it could be shown that she did work there and that he signed the yearbook (despite questions I have about that), that still does not prove that he did any more than visit the restaurant and sign the yearbook.
Yes, disproving the employment (especially) or the signature tend to indicate the rest of her story is bunk, but even the absolute refutation of those aspects does not address the base allegation. It becomes she said/he said, and despite her credibility being severely damaged at that point, there are still those who will believe her (partly because they want to). In order to prevail in civil court, he has to prove innocence, that he did not do what has been claimed, or show she has a good reason to falsify her account, one which the jury would find more credible and compelling than her claims of wrongdoing.