Imo that doesn't matter, whatever you might feel about Ethan or Erynn or whatever else, she IS your childhood friend and more importantly, she IS your mom's best friend daughter and your neighbor, I mean shit, you'd intervene if only for Melissa's sake...Remember the history with Ethan....remember what he has done to you personally...remember Erynn's gaslighting, withholding information, secrecy....not a dick move at all to think, I'm 18, she made her bed, why I should get beat up again. (I say that as a huge Erynn fan btw)...but as Def said, if you don't help her, you never see her again....she literally disappears out of your life (and the story) because you are no longer of use to her and/or helping her...
Think about that....
So yeah, I think the reactions to you not helping her are not only justified, but probably not as bad as they should be, tbh.