The Philly's Pissed group wrote some zines on supporting survivors that you might find helpful.
1. Support Zine - http://www.phillyspissed.net/node/18
2. Supporting a Survivor of Sexual Assault: a Primer - http://www.phillyspissed.net/node/13
3. Thoughts About Community Support Around Intimate Violence - http://www.phillyspissed.net/node/10
Rape culture is the worst shit. I don't know your friend's situation well enough to give advice with much certainty, and as a male I have a different lived experience, but, perhaps discussing with them about the concept of prior informed consent, acknowledging their understanding of the situation, the concept of victim v. survivor, and asking them what they need from you, would be useful starting points, leading up to more discussions and if they find it useful then something empowering such as a public call out of this person if their identity is known, physical vengeance, learning self-defense, or whatever makes sense to them.
An anarchist's response is to empower the disempowered and disarm or destroy oppressive dynamics and individuals, to do what must be done as defined by the survivor and the context of the situation.