Yes exes can be friends. I don't know how much you can call someone a best friend when they live several states away and you've experienced some sort of relationship break down with them in the past, but I am friends with some exes and my SO is friends with damn near all his exes to some degree. We've even offered one a place to live till she found her own place when she was considering moving to our area..
Anorexic? You say that like it's a bad thing..
Because BPDers have such a weak and unstable self image, they absolutely HATE to be alone and have a great fear of abandonment. It therefore is common for a BPDer to create "explosive fights" and abuse her partner without wanting him to leave. This is why the #2 best-selling BPD book is called I Hate You, Don't Leave Me!.