You saw an injustice and felt called to speak up, so you took responsibility and stood up for what you believed in. You did so even knowing that it would be uncomfortable and might cost you a friendship. That was really courageous.

It's normal to feel like shit when you find yourself in a scenario when all the options seem terrible, but your actions show that you have real integrity, and it's okay to pat yourself on the back for that.

It's also okay to mourn the friendship, it's okay to recognize the effect this may have on this person and feel for them, and it's okay to take time to process your feelings about the whole thing; however, I will caution you to avoid delving into self-deprecating territory. Any consequences that they face for what they did will ultimately be because of their own choices--not yours.

Just to drive it home: you aren't getting them in trouble. They're getting in trouble for what they did. What you did was trust your gut and call out something harmful.

Know that everything you're feeling is totally valid, and please be kind to yourself and try to remember that you did the best thing you could in a difficult situation.

EDIT: I wrote this without knowing the details of what happened, then I read your comment about what happened and I am doubling-down hard on what I said. You def made the right call, you are brave as hell for pursuing that, and the person you reported needed to be stopped. It's understandable why your friend didn't press charges (though I personally believe that to be a mistake), but you did right by her and this dude's future victims. It's okay if she doesn't see that, and it's okay if she's upset with you--she's clearly in a very complicated situation right now and her feelings are valid, too--but please know that you absolutely did the right thing.