Hard to believe Cassie didn't know about the tape

Ya, that was a tough sell. Videos like that don't stay private, AND she was in that circle of friends.

I disagree. Cassie was focused on her studies. She was oblivious to what was happening with the more party-hearty parts of her college. I've been in a similar situation where stuff has effectively happened under my eye without me noticing, because I didn't think it was my place to pry. And such a tape would remain a secret among the people involved, because no-one involved, including the people who watched the rape, but weren't personally taking part, would want to be associated with such a terrible crime.