Yesterday, 3/27/18, it was reported that the porn actress Stormy Daniels sued President Trump's lawyer Mr. Cohen for defamation because he alleged that Stormy Daniels did not have an affair with Mr. Trump.
If not, then how can they argue a defamation? Even if the allegation of the affair never having taken place is false, it is a lie which does not damage anyone's reputation (ie, it does not "defame" anyone). In fact, even if it is a lie, it seems like it is a lie which can only serve to improve someone's reputation.
Is there any precedent or law which recognizes a claim of not having an affair to be defamatory? Is there any law or precedent to make a claim that someone lied publically (but not under-oath) a defamatory claim?
Edit: this answer below shows that the claim of defamation stems from allegation that Stormy Daniels was called a liar rather than from the allegation that she did not have an affair.
In a similar vein, hypothetically, if Mr. Cohen made a public statement that Stormy Daniels was lying about being or having been an adult movie actress, would that be defamatory?