My friend slept over at her friend's house and got sexually violated?

Ok so my friend, who's only 14, had plans to go to her friend Katrina's(Name used to hide identity) slumber party. There was at least 7 or more girls, by what she told me, that were also invited. Anyway they stood up way too late and passed out around early morning. Now Katrina has 2 older brothers whom I have in a few classes, The oldest is 17 and the other is 15. My friend Lexine(Name to hide Identity) woke up and felt her face, she said it was *** and that it was all over her. She found her shorts and underwear cut on the sides, She brought them home and called me to come over to show me. IDK what they cut it with but the shorts didnt even feel that thick anyway so it mustve been easy to cut. She hasnt told her parents or older brother about this incident due to the fact she doesnt want her friend Katrina getting in trouble. I was so pissed off I threatened the brothers at school and threw a few punches but I know that's not going to change a thing. IDK what to do and it's killing me not being able to tell her parents and brother. I need HELP!!