Technically, you just said, you dont even have to report rape, cuz it "doenst matter anyway".

No, that's not what I said. It's simply something which is ALSO impacting a victim's decision to move forward. We have no idea where OP lives and we have no idea how her environment will react.

For taking care of this guy myself as soon as he is out of jail, for example or just for following:)

And how would have that helped your friend? Just accept that this is more about your feelings than hers at that point.

Do you even know how it is to live with the thought, that maybe your closest friend raped her or "just" this random guy on the street?

Yes, I do. As I said somewhere else, stranger-danger is just one tiny part of this huge problem. Many crimes get committed by fellow family members or friends.