This girl needs to get cut out of his life, or it's going to poison your relationship with mistrust. I'm telling you this from experience.
My bf's ex wasn't so psycho and aggressive, but she 'made sure' they maintained a friendship (which for awhile I went along with) so that she could have opportunities to try and 'win him back'. Eventually I had to draw the line, so I told him, it's her or me - you either cut off all contact with her or I'm walking. So he blocked her number from his phone, he blocked her email. She phoned once on a different number, and I just happened to pick up his cell and I basically said "If you want to tell him something, you can tell me and I'll pass it on if I think it's important." She didn't call back.
Just make it clear that her psycho behaviour is making things difficult between you, and he should be making all efforts to get her to back off. He should not be picking up her calls, and if she does call, he should just hang up on her - why would he WANT to carry on a conversation with someone like that, especially when he knows it hurts you?
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi