I've never not helped Erynn, so didn't know that she calls Alexis if you didn't help.Maybe she will mention something to someone like Alexis or Iris... she does call Alexis for help if you leave her in the alley.
And that will lead to this group of people learning what's going on.
One of my earlier random theories before the sped up blackmail plot, was that Santi / Zo might get involved. At the club where Ethan meets John Ethan is excited when he thinks John has contacts, particularly with the basketball team. John obviously doesn't really have those links, but Santi does... The groundwork isn't there for it to work any more, there's too little time given than Santi and MC haven't even had their father / son chat yet. But Santi could actually have made the John plan work with Alexis helping...
Anyway, much simpler just to have John beat Ethan to death and get it done!
Taking her comments with a massive pinch of salt, but hasn't Nyx said that she maintains a private alternative version of the game that includes stuff like this? One of the examples was that in her personal version the 'fake erynn' in the last episode actually was Erynn if you weren't helping her, along with a few other sort of ntr things she's added. Seem to remember her saying that none of that content would ever be in the official version of the game, but she might release her alternative version once the main game is done...Let things stay as they are, and Erynn gets a bad ending... maybe as Ethan's mistreated GF, maybe embraces being a whore(like the old corruption storyline that got cut, except you are not an active participant).
Doubt Erynn will get a really bad ending in the main game, would just be too dark and wouldn't go down well. Can see one last chance for the MC to do the right thing, and if not some combination of John / Alexis / Melissa sort it out while the MC ends up deservedly hated!