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I think the biggest thing you can do is listen to her and let her know you're around for her and understand what a big impact this has on her life. It's sad that her mother isn't doing this for her and isn't acting in her best interests. It sounds to me like her mother isn't accepting what happened and is trying to sweep it under the rug and just pretend everything is fine. You could try to help her get into counseling as well, even if it's a school counselor or someone else. She sounds like she needs to talk about it and work through it some more.

Based on your age, I'm assuming hers is the same and she can go talk to whoever she wants about it, with or without her mother's blessings. And if there are other small children around him, I think she should be speaking up more and helping ensure he can't hurt anyone else.

I understand that it's overwhelming for friends and they don't always know what to do. But just listening means so much.

Not only should she report the grandfather, but she should report the mother as well. That mother knew what was going on and protected the abuser rather than her own child. That's absolutly disgusting!

If this isn't going on any more, I am inclined to say that she should wait until she's old enough and ready to move out on her own. I say this because, while personally I don't think "moving away from friends" is a particularly traumatic thing to do, I know that many people feel very differently. If she waits until then, she could at least stay in control of her life and choose where to live rather than moving or being put into a foster home or something. And, certainly, having close friends by could be a huge support network.

But either way, she absolutly has to tell someone. As far as I am concerned, the mother and grandfather are equally responsible in this.

I'm guessing the Grandpa is the mothers father? She is trying to protect him but he doesn't deserve that! She might know what happens in prisons to people who hurt children in anyway, it's not nice. If you ask me he deserves to be punished!
The best thing you can do is respect what she asks you to do and just be there if she needs someone to talk to. She must be feeling lonely becauses of what her mum said.