There you go,if you ever had chance then this is it,go to her,find some spot you could talk to her 1on1,make sure you keep eye contact with her,don't look down,keep eye contact with her all the time,if she does the same then you got good chance to get her,then tell her you have some bad news for her,which she should definitely know about,then she says what news?(Tell her that as soon as possible because that kind of stories will leak really quick)
Tell her, well i don't wanna hurt you or make you feel bad but i must tell you that because i think kind of girl like you shouldn't be treated like that.Then you tell her that story,she probably gonna cry or sadface,(if she really cared about that guy)then you tell her i'm sorry and hug her instantly(Don't expect a kiss by the way)then you tell her, he didn't deserve you,then what happens next i cant really predict.
Also be confident and calm,don't overreact,just be smooth.Also from my own experiences when you don't tell girl you like her then nothing is gonna happen,because we males must take a first step unfortunately,and trust me most girl like when boy tell her i like you,it makes them feel good and desired .So don't be afraid and do it.
Good luck man!