It may be impossible for a person being accused to prove he did nothing, but I still am concerned that if Herman himself is not able to offer some degree of proof he was never in a position to have done what has been alleged, Mrs. Cain will not be able to resolve this no matter how nice or believable she is.
Trying to look at her interview realistically, the wife is often the last to suspect or to know as it does require some split personality to fool around and a philanderer will go to great effort to keep their spouse from knowing.
How do you disprove a negative?