Okay, that's easy to do.
Erynn's big mistake was that she was already fully aware that Ethan bullied the main character and yet she continued to be friends with Ethan.
I guess it's a matter of perspective. As far as we've seen, we can't be sure if she was fully aware of that or not, can we?. The way I see it, she only knew Ethan had been kinda mean to him some times, but not to what extent. The relationship Erynn and the MC shared implies, to me at least, that had been Erynn fully aware of all of it, she'd have cut ties with Ethan for sure.
Ethan was just trying to act cool, macho and grown up in front of Erynn, encouraging her to ignore the MC and be with him, instead. That's hardly bullying him. Erynn could've been very good friends with the MC, but I can see why she'd also want to have friends of her own age, to hang up with. She might've wanted Ethan and the MC to just get along with each other, instead of fighting.
This scene we saw of them all was borderline for her. Had it continued, she might've had more proof, and the story might've developed differently. Unfortunately, the MC pretty much became a shut-in, stopped talking to her and, shortly after that, he moved with his father.
Honestly, the only thing I can blame Erynn for is leaving the MC without any answer after his confession to go with her friends. And even that is somewhat understandable, as she got embarrased by it, which would be a normal reaction.