Wow, that went from 0 to 100 fast. There are not many flashbacks regarding either Erynn or Alexis, so your need justify having receipts is pretty thin. Not reading into what may or may not be there, as it is never stated, we can only surmise.If YOU listened to their chatter you would know they didn't have many interactions and were nothing to each other before that day, she wasn't into him, him leaving town didn't affect her...
And she was angry about the kiss back when it happened (now, after 5 years, she thinks it's cute, but she didn't like when it happened), i have the receipts if you want.
Her obssession is completely out of the blue for a guy she barely knows and she's willing to withold important information from him and lie to her BFF to keep it going, she knows she might be causing conflict but she just keeps moving forward.
...
She's a cameo at best if you passed on her.
For a character that is Erynn's BFF, Layla's sister... she's remarkably uninvolved.
...
Erynn handled the entire flashback very poorly. Was she surprised by the question? Sure. Did she need to turn and go back to Ethan to laugh over a social media post in the middle of that conversation? That's a very poor thing to do. Was it smart to verbally spar with Ethan instead of checking up on the MC and resolving the question she left hanging? Not at all. Sure she tried to defend the MC, but he doesn't know that (we do) and even then is that the priority? Why not go find the MC and talk about his question and see how he's doing?
The catalyst to the whole thing was a misunderstanding because of Erynn. Had she just gone to the MC instead of Ethan, the whole him leaving town would likely have never happened.
The fact you see Alexis as angry, is surprising. She had no anger response at all. Her facial expression was of surprise and shock, and her thoughts were questions and not anger/accusations. Alexis' response to check up on the MC was by far the most compassionate moment of the entire flashback. Erynn didn't exhibit one compassionate or romantic moment in that entire scene to the MC.