She is not the one to blame for this situation (Cuz she was just having sex with her now ex boyfriend and yada yada) because ethan is cleary the one to blame for what he did.That's one idea that could work. She didn't even think about that just like she didn't even want to see the video when MC confronted her about it. She imidiately jumped to conclusion that if Zane knows Ethan and they're close it's probably that they work together. Like she didn't care to hear Zane at all and see what's the deal and is Zane really informed on what's going on, that he's on sex tape with Erynn. No she imidiately bows down, ditch Zane and goes to prom with Ethan. And instead of deleting video as part of a deal he used it against Erynn to se how far he can go with her. And honestly the only person to blame here is Erynn herself for not telling anyone and for allowing Ethan to blackmail her.
BUT She is the one to blame for this problem to last that long. She is a moron for not seeking any help or do something against it, like you said. An audio recording of Ethan saying something about the blackmail