Wolferine she has probably tricked a lot of people to try and kill Kematu that's why if you help him he says
" Not everyone can be seduce by you". BlackArrow has some extremely good reasons to help Kematu
They're not persuasive reasons given that most of what he relies on is based on the incorrect presumption that the Redguards and the Aldmeri Dominion are still at war or at least were recently still at war. Some of his observations are simply incorrect. There are no "Hammerfell official uniforms" and Kematu is simply wearing standard generic Alik'r garb. The Alik'r are
mercenaries and Kematu openly acknowledges that he's a mercenary who is offering to split his take on Saadia's bounty with you. The Alik'r mercenaries harass every female Redguard citizen of Skyrim to the point of almost kidnapping them, and they never apologize to any of the women they question. Instead they make a threatening warning to every woman they question to stay out of trouble. Kematu's dialogue is only indicative that Saadia is manipulative not that she did what he claims she did, which, again, is an incredulous claim given the timeline.
It's far more probable that they are both lying. It could have to do with political intrigue in Hammerfell among the noble houses as Saadia is descended from one noble house and Kematu is supposedly under the hire of another noble house. We'll never really know with the limited information we are given but, like several choices in the game, it's designed to be morally ambiguous to allow players to role play it either way.