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I don't know what US state this was alleged to have occured in, but the statute of limitations for sex offenses were much shorter 20 years ago than they are now, and it's the statute of limitations in effect at the time of the commission of the alleged crime that controls, rather than the ones of today. But also, even if the statute of limitations hasn't already run, the complainant's testimony has already been greatly discredited. The two polygraph tests, the admission by the complainant herself that she had lied, and the dismissal at the time of the so-called incident, makes for strong evidence in your father's favor that the complainant is lying.
I also don't know if at the time the case was dismissed with or without prejudice. If dismissed with prejudice, the state cannot refile the charges. But assuming the state could refile and the complainant could convince the police to charge him again with this, there is plenty of evidence that would yield reasonable doubt. The odds of him being convicted are very low.