I agree with your answer 99%. My answer would have been the same, the power dynamics that prevent someone to talk is the same whether you're the victim or you're what we can call an innocent bystander.
There is a difference, though. If it were me, I honestly don't know what I'd do. I'm convinced that living a trauma, like a sexual assault, and seeing it from the outside and doing nothing are different things. Knowing that you could prevent someone else from suffering and that, by not talking, you're actually being complicit in a crime like that is not an easy decision. Of course it's too easy for us to talk, we're not involved, it's easy to spread blame. Still, I can't help to feel a little troubled when I think of the amount of people that know and do nothing. I'd rather know that at some point, someone would lose the fight against their conscience and talk.