Yeah sometimes we do that, when our weaknesses show in front of a person, sometimes we avoid facing that person so as not to feel embarrassed or pitiful , but Chan il saw Ri-in panicking when Cirrus called, she even forgot her bag and left the place in a hurry, that would make anyone think that there must have been an emergency or something, then he saw her with Cirrus who was crying his heart out, so instead of thinking that something really bad happened to his friend and running to check on him, he only thought he was trying to steal his girlfriend!! Even if he was avoiding him, why didn't he just call Ri-in to check on him and learn what was going on? Instead he just accused him of cheating! I am sorry but that doesn't make him a good friend in my opinion
Before this incident Cirrus has been treating Chan il very badly, so I wouldn’t blame him for thinking things the wrong way. He tried to clear it up with Cirrus, didn’t even assume things. Cirrus purposefully didn’t say anything because he knew this would result in Chan il feeling betrayed.
They’re high schoolers, you can’t expect Chan il to be an angel when Cirrus has been treating him like shit for a while now. Chan il gave Cirrus the benefit of the doubt and Cirrus didn’t bother trying to clear it up. Realistically, anybody would’ve reacted like Chan il, they’d actually have cut ties with Cirrus wayyy before if we are being honest. He’s been clueless from the beginning and nobody wants to give him insight, so how was he supposed to know?

It may be an unpopular opinion but I don't like both Chan il and Ri-in, they're both bad friends that I would never want to have, especially Chan il, who knew exactly what Cirrus was going through yet he didn't bother to ask what was going on or be there for him even after learning the truth, and Ri-in, yeah she was there for him when Candy got sick, but why is she angry with him? I understand that she might have felt betrayed but it's not like he FORCED them to be together! They simply met and liked each other, that's it!! It's not like he ruined her life!! And she got back together with that same guy!!! That's why I really don't like both of them