"She says... when they first started coming after her, the Border base wasn't built yet. Nobody knew about Neighbors. So when she asked for help, nobody took her seriously. And then... there was one friend who did."

he very explicitly excises his own memories of that time from the story he tells osamu. so we Don't know about what rinji did at the time, how he felt about it, any of that. and in chika's firsthand account of the time from chapter 179, she Also doesn't mention Rinji specifically.

"My mom, my dad, and my classmates... none of them believed me. None of them except one friend."

although when she tells yuma about her situation she says she told both of them her secret. so thats there at least

to me, the most interesting read on the situation is that he didn't believe her either at first. maybe he changed his mind after aoba disappeared, or maybe in the wake of the first large-scale invasion and Border’s appearance, when ‘monsters who kidnapped my sister’s friend’ became a story with a lot more people who could tell it. either way, it’s clear that he suggested that chika get help from border, and she refused. it’s also clear that he doesn’t think much of their parents. 

“We’re the only ones who can look after her.”

“We might die or get captured. Your parents would never forgive me if that happened.” “But that goes for you too...” “It’s not the same.”

(admittedly, he’s talking to a fourteen year old kid: in a very real way it isn’t the same. but that’s kind of belied by telling that same fourteen year old kid that he and rinji are the only ones who can look after chika, huh!)