You hope that last and could never know that.
First, my take on Erryn is that she is overall a great sweet girl as already shown by MC in plane where that dichotomy with what happened at confession in MC's mind was already clear from start on who she was, if not perfect and possibly hurt by all that happened due to an asshole in her life, misfortune, too many secrets and a few bad decisions.
Considering fact Erryn might not have known Mc was mad at her and should have not known about that and was completely clueless and unaware. Yes, it was not her intention to hurt MC and it has never been. Also not with her more or less ghosting or paying les attention to him before while growing apart, which some think was just new friends and I think a bit more. Idea that she might not immediately have connected his changed behavior to what happened at confession (which was also his birthday, so nothing was done for that after should have also be seen as a bit weird like she had no present for him), I can get from her pov. That she still did not have at least a suspicion or a niggle of doubt that something is amiss and might have been taken other as she intended a few weeks later though after him ghosting her and all the drama going on next door is a lot harder to swallow for a girl that can do University and is a golden girl.
Like said she was 16, not 8.
Just a few facts now though. She was not a stranger living 20 miles or more away from him, but next to him. Her own mother was like a second mother to MC and the main emotional support and friend of MC's mother. Now that guy for some reason becomes such a terrible terror within a few weeks after that happened that he is suddenly being sent away (MC did not leave cause he wanted, but was sent away by Emily and that was the reason he did not accept contact with Emily though either likely Andreas fanned those flames.)
Now either I have to assume that a few weeks of MC being hard to handle would be just a send him away withouth any feelings on the part of Em and Mel and something they never talked about or tried to find out why he changed or that Erryn somehow missed all those talks, musings and pain those women likely went through. That while Erryn at 15-16yo would have been one of the most logical people to ask and talk about some of the questions those two women had and was easily available to them to talk with. She was not 9 yo that you might think you would have to fully protect her and not be asked about her opinion even if they might not have listened fully to her. Same as that we know Erryn now is aware that Ethan bullied her friends, we do not know exactly when in the timeline she found out, but well if she did not suspect MC would have been a victim of that bullying as well that would be strange.
So either I have to believe it is believable that Erryn somehow never was a part of those conversations or otherwise noticed something was going on next door, Mel never told her MC was sent away, also after weeks of ghosting her and ignoring her correspondence would not at least have an inkling something was different from his side. Remember we are talking about a sweet girl who did care about MC and both Mel and Em knew this, just sending him away without telling her would be a betrayal to her as well.
Now I have set out before a situation where that might be somewhat believable, we have all been in those first few weeks of a new relationship where we somewhat ignored and neglected the ones we cared about cause we met a girl with beautiful eyes that was for while all we could think about and then later normalized that situation and integrated her with our other loved ones and friends if the relation continued for a long while. Now this would fit with all that happened, would make Erryn noticing less way more believable and no less sweet.
The other option is that I have to believe that Mel and Em were so self absorbed and within weeks went, we have to get rid of tht terror and sent him away. he is troublesome and we do not want to handle that or put any time into finding out why. Let's get rid of the annoyance and let us also not tell your daughter Mel who is his friend and cares for him like a little brother. Well that is not the way either Em or Mel have been portrayed, but it would explain why MC did not feel like talking to his mom or anyone for 5 years if that is how he felt and actually was what happened.
Same that a lot of people about the Alexis claim and the guilt Alexis may feel are talking about things like Alexis ghosting Erryn at the concert for MC, which is less nice of Alexis indeed. The thing is though that the guilt of Alexis is already there in the plane and after MC an Alexis meet first time in bar after plane without MC knowing she works there. Where there are no other reasons for her to feel guilty as her conversations with and confidences of Erryn. Well weirdly my take is in a way that the situation between Erryn and Alexis is a weird mirror to that of Erryn and MC where they are still friends that grew apart and one fell for the wrong person hurting their friendship. Just hoping this will turn out better for all involved and that they might find healing, love and have eachothers back as friends.