Before you decide to go vigilante on someone who has traumatized a friend or acquaintance (or anyone), it's best to ask the victim what justice looks like to them.
After I was raped, the *worst* thing, and it happened over and over again, was when I told someone and they said, "I'm going to fucking kill him." It didn't make me feel taken care of-- it made me feel like even those I trusted were capable of horrible violence and unsafe.