I think that it was good that you have a place here at PC to vent this anger and new challenge you are having emotionally rather then not having a place to vent and put what you are feeling into words.

Often the words struggle to come forward to articulate what something like this meant to oneself. However, seeing someone else be a victim can really bring a lot up emotionally that a person could not articulate before but suddenly finds themself overwhelmed and angry as you are describing.

This friend is probably mostly stunned right now and doesn't even have the where with all to even begin to articulate what just happened to him. You can't fix or change what happened, and you may be too emotional to know how to reach out and support your friend right now too. And in the situation you are describing the person who violated was someone both you and your friend knew and may have trusted. Ofcourse that would be a big challenge to you as well on a very deep level.

If you find a need to comfort your friend but don't know what to say at this point, that is understandable. If you do reach out what you can do is validate that your friend is stunned and that you understand that he may be very confused right now and not know what to think or how to feel. It's ok to let your friend know that you are sorry, that you are angry and you want to be supportive to him and that you want him to know that it may take time for him to figure out his feelings and it's normal to be just stunned and speachless.

It is important to know that the individual who is "wrong or dysfunctional" the most is the abuser. Often these individuals never show the part of them that can abuse like this so it's never the victim's fault and often is not something the victim knew to be careful of or was a potential danger. It sounds like neither your friend nor yourself noticed anything that said "this is a bad person" in this other individual. I think it can be helpful if you make sure this individual is not the only one that did not see any warning of this so this victim isn't alone with thinking it's something he should have known and begins to feel guilt about. I think that it is very helpful in a friendship is when a person is not alone in being caught off guard instead of being left alone with this and begin to feel guilt about it.

Also, in this scenario, from what you have described, you are a victim yourself because this individual was supposed to be a "friend" and what this individual did was yet again make you yourself question your ability to trust. Something like that does a lot of harm to one's own "self trust" and that is often very hard to articulate as well as slowly work on gradually rebuilding. On some level you do know this yourself, and that is part of your own anger and emotional challenge with this scenario. However, often the abuser is the one who has the most by the way of "emptyness" and has no where with all to accept any trust or friendship, so much so that the only thing they do know is violate and often it is these individuals that are worse than anything you experience emotionally as they already consider themselves as "rejects" as either they are born psychopaths or they were so neglected that they genuinely don't know how to respect or care.