Libel per se relates to damages. A plaintiff does not have to show damages - the statement is presumed injurious enough IF IT IS NOT TRUE and meets the other elements.
There are a boatload of reasons not to bring an action against Arnold on this besides the fact that, well, there is no case. The case would publicize the woman't arrest record even more than it has been - though her reputation probably isn't on her mind. Defamation cases are notoriously difficult to win and you usually have to pay out of pocket to get someone to represent you (though here there are political motivations which would probably make some liberal represent her pro bono). When she loses - and she would - the public will just conclude that the charges were true (David Irving in England had this happen to him recently and lost everything). I also think a court could conclude that this woman, raising her claims in the heat of a campaign, became a "limited public figure" for purposes of this issue.
Let's just say I wouldn't mind representing Arnold in any such action and that bringing it would be foolish and purely political.