Dr. Abdic is one of many characters that defy our expectations, which are set by Rachel's first impressions and overactive imagination. She sees Dr. Abdic kissing Megan, whom she still thinks of as "Jess" at the time, and Rachel's anger at Jess's infidelity sets her plot into motion.
However, this is like an episode of Three's Company with violent consequences. It's all a misunderstanding, see, and Megan is simply giving Dr. Abdic a friendly kiss. Yes, Megan wanted to have an affair with Dr. Abdic, and they almost crossed that line, but he stopped her. Unfortunately, Rachel's rumor causes Dr. Abdic's reputation to be dragged through the mud. We'd definitely read a spin-off novel, The Therapist of the Girl on the Train to see how these false accusations really affect him.
Dr. Abdic's a good therapist, so he'll probably be fine. Both Megan and Rachel see him at separate points, and both women have ulterior motives—Megan wants an affair, and Rachel wants to see if he's a killer. We have a feeling that Dr. Abdic would give this kind of advice to both women: "I'm not sure that sounds like a very real kind of happiness. […] Not the sort of happiness that can endure, that can sustain you" (16.35). In other words, he's the voice of reason in a book full of suspicion and misdirection.