I never saw issue with Zane not knowing. Unless Ethan told him about it which would be dumb unless he is involved with Ethan getting the tape. I don't see how he would find out. Erynn made some bad decisions so if she didn't tell Alexis until recent who is her best friend, I don't see her telling Zane. I personally think it makes more sense this way as at least Erynn would remain consistent in her bad decisions.
Yeah, thinking about it, it all depend on how the video got filmed, what Ethan told Erynn and when he said it to her. If she was already broken up when she found out about the video and never talked to him since then, it COULD make sense. Especially since she doesn't make many (or any) good decisions about this situation until she tell MC (and she wasn't even planning to).
But it's still 2 bad scenarios: Either
A) she think Zane is involved (filmed or knew about it) and would talk badly about him when she explain the situation to MC-Alexis-Melissa-Emily-John, which would make him a very bad guy in the eye of everyone. Any of them should be massively angry at him when they will face him if they think he was involved somehow.
EDIT: and they should want revenge on him too (at some point).
or
B) she knowingly keep this information from him (which we established it could make sense), but nobody that heard about the video had the idea that he might deserve to know about it. Realistically, if Erynn doesn't put ANY blame on him, at least one of them would've said something about him deserving to know that this video exist.