Harmanuel Kendall has 3 class levels in Korvosan (an archetype of Expert), which gives him a +1 bonus to saving throws against discombobulation due to willful and knowing slander. Unfortunately, it's a very high DC.
In five minutes he'll - l'esprit d'escalier, as they say in Galt - determine that what he should have done is angrily threaten to sue, and demand that his legal fees (and truthspell) be paid for by the loser.
It would have been a fantastically Korvosan response.
Instead he's going to get indignant in a way that
They consider confidence and competence the greatest of assets, and they deride or heckle those who display weakness, indecisiveness, or inability.
damages his credibility.