There are three very strong reasons why some previous victims become perpetrators. One is mentioned above.
Just like all of us, victims can become under the influence of some pretty unhealthy people. Whether that is in a group or with an individual the controls are the same. Unless we learn what those are, we remain susceptable to the crazies around us everywhere.
Think of it as switched addictions. We learn to sacrifice who we are for the rules of another person or group. What I read of the article was that she did not participate but was present. For all we know she could have been tired up and forced to watch. But even if she was being "forced" to watch she was present.
While victims are being abused (as children) they learn to go to another place in their heads. They freeze because as a child they don't have a lot of choices. Fighting or running away are pretty much not going to happen. So they freeze. In that dissociative state they cannot think about what is happening. They think about anything else except being there. The more often they are abused the stronger the dissociation so they do what they are told and block out what is happening. Not the same as forgetting. Some do and others don't.
As they get older dissociating becomes a way to deal with anything you don't like. Just space out and pretend whatever is happening isn't really happening at all. So in a situation like this it is possible for the person to stand there, perhaps follow simple instructions and really ignore what she is seeing. She becomes incapable of taking appropriate actions and protecting the new victims.
There is another option and this one is frequent with male victims but rare with women but it does happen.
Abusing another person is a way to get the power you didn't have as a child. Remember sexual assault on anyone is not about the sex. it is about power. The man clearing wanted power and was using someone else to get it. If he was also controlling Chessa he doubles the power he is stealing from others. If she is participating in any way then she too is trying to capture power from the victim and align herself with the male abuser. In cases where women become sexual abusers there is often other mental health issues stemming from previous abuse but possibly other issues as well.
I would pretty much assume the male abuser was also a victim as a child. These are NOT excuses. They are possible explanations for behavior.