Disclaimer, I only did one play through with some back and forth to test different outcome, but I always tried to ignore Erynn. I did it because the whole premise is about a guy that had a crush on a girl and that she did something bad enough for him to decide to go away for 5 years to ignore her.
First problem with Erynn, is a writing problem, there is 2 flaws to her character. Like a lot of people already state, you never see her as a good friend. If you try to ignore her from start, the only good thing she's doing is letting you sleep on her couch, I would have prefer to go somewhere else anyway. You know from comments(Mom, Melissa) that she was a good friend and MC acknowledge that she was before the 13th birthday event, but you don't see those moments. The MC said he had a crush on her, but you don't really see any building toward that crush or why he had a crush on her. It's something you should take by default. Being told someone is good will never have the same impact has seeing a scene were that person is good.
In defense of the dev, I understand it's not something easy to establish a character with a past with an MC if that past is not really part of the game. I think if the game started with the MC in plane thinking about his friendship with Erynn in detail(multiple scene building the crush of the mc to Erynn) until the 13th birthday. The moment the player will see her for the first time, there is a bigger chance that the player want to speak to her to get an explanation. That a lot of work and I'm not sure a lot of people would have cared so much about it, I mean reading the thread, Erynn is one the favorite LI, so maybe the dev was right not doing it, but I think the depth of her character/firendship suffer from it.
Second problem, the dev seem to want to paint her as a damsel in distress archetype LI. the problem is the situation is lacking because like many others told before, the blackmailing could have been resolve pretty easily in the past. Come on, he told her he would delete the sextape if she went to prom with him and he didn't do it. Why would you indulge him in the future, you know he will never stop. She should let the sextape come out, I mean who care, she's fucking with her boyfriend of the time, nothing crazy. She was not cheating, she was not part of a kink that society will not accept. All that is explain by her being scared of not being able to become a teacher if that came out(really?)...So she think it's better to be force into being an escort(non-sexual)and being abuse then loosing a chance at her "dream job" of being a simple teacher?
Another thing about her, after the 13th birthday, the MC ignore her and go away for 5 years without any attempts of giving news to her, taking her calls or replying to her letter(s?). For some reason she still expect that the MC will come back and be her best friend. Even after learning from Melissa that the MC wasn't interested to meet her, she still expect him to drop his friends to spend some time with her at the club. This is something that I find annoying about her. I do understand that she want to build back the relationship, but take you time, the guy isn't ready yet and she should have understand it by now.
The way the story is told is problematic too, I like non-linear story telling in movies and book, but in interactive story there is a problem, because you end up taking decision without knowing thing you should know. When you are in the plane and decide to fuck Lexi, you don't know that you were in love with Nikki(my play through) at that moment. So when you end up explaining to Erynn what happened in Riverside, you end up like an asshole if you went for one of the girls... because it's mean you fuck the first woman you meet right after leaving the one you loved...but you didn't knew that when you took the decision of fucking Lexi. That not a major problem because I don't remind any other occurrence of such later in the game.