Crazy girlfriend of my bestfriend's ex is still having a grudge on her. freaks me out too!help!?

few months back,my bestfriend hooked up with a guy not knowing he already has a girlfriend back in his hometown. as soon as she founds out on this, she immedietly backs off but the girlfriend already knew about the boyfriend's betrayal so plans her way to ruin my bestfriend's life when its not my bestfriend's fault. if she'd knew the guy was a liar, she would obviously can get another man cus she's pretty and kind in heart. knowing all this should stop, my bestfriend put up some courage trying to explain and tell the crazy 'gf' the truth hoping that she wouldnt intefere her life any more. at first she was ok but recently, i read the 'gf's blog expressing her angst and plans to destroy my bestfriend and she even reveals my bestfriend's idenity. although she wont find my bestie but i felt devastated and i know that if my bestfriend knows about this she would cry her hearts out. what should we do now?