How to tell my good friend I don't want to be introduced to her psychotic friend?

[Jacinta]

My good friend amy has a very close friend called Elspeth that she would like me to meet. However I don't want to meet her. Not just now, but EVER! The reason for this is she is a complete nutter with what seems like deep rooted psychological problems. I have never met her but my friend says things about her which she thinks are great but which leave me distured. Elspeth claims to see visions of her regularly and says that even before they met she knew she would come into her life. She also says she had a vision of her and her boyfriend before she even knew any of them. I don't necessarily think Elspeth is lying about these visions, I think maybe she does think she has them but either way it is disturbed and abnormal thinking. Even if she was normal my friend has explained that Elspeth rings and texts her every few minutes and gets annopyed when she says she's going out with other friends. She seems very jealous, paranoid and controlling. I asked my friend if she would like to go for coffee and Elspeth replied to my comment when she doesn't even know me saying that my friend couldn't come because she had a lecture to which my friend replied saying she could come because the lecture was cancelled. My friend is Elspeth's only friend and she has been trying to get a job in our city so she can be near my friend all the time (clingy). She just seems a very distured individual and I want NOTHING to do with her. How can I tell me friend nicely, that while I have no problem with her being friends with this girl, I do not wish to meet her or become friendly with her?