Quite simple.

If it's YOU she's saying that raped her, then by all means kick up a massive stink and let the world know you didn't.

If it's someone else she's saying that raped her then tell your mate quietly that she's a fucking nut-job and then leave him to get himself out of the shit. Then he'll appriciate you as a mate. But if you try and split him and his burd up he's just going to end up hating you.

Let him make his own mistakes dude.