Mari must really feel lonely if a friend talking about boys changed her entire perception of that person. It's interesting how Mari just spontaneously talked about love. Both Kokon and Kengo never expected that, and thus had to adjust on the fly to do something about it.
Mari must have had some real issues with betrayal in the past, but how come? Did her father betray her mother? Did she never get bullied by fake friends. Seki's right that she probably would despise someone like him, who would fake his personality to get close to her.
I wonder what Kokon found about Mari that made her realize her job was hard?