When a very close friend is not there for you when you need that person the most, this is very hard to forgive. I understand wanting to keep a friend who has been in your life for such a long time. I know that my BP existed long before it was diagnosed. She knew you for all those years, and your BP never seemed to make any difference back then, so why are things so different and difficult for
her all of a sudden???
If anything, I have become a lot easier to deal with since I was diagnosed, and have been working on it, taking my meds, etc. The fact is, she liked the undiagnosed you. Now that you are dealing with this head on, your life is going to improve. Mine sure did.
I have to say, I have known quite a few individuals with BP throughout the years. They are healthy, successful people who contribute a great deal to their friends, family and society in general. Your friend seems to know very little about BP, and the prognosis when treated. This is really unfortunate as a social worker (I know, I keep repeating this, but empathy is essential in her line of work).
In my experience, NPD individuals make everything anyone else is going through all about them. My sister has screamed at me several times because she thinks my BP is all about her, and I have been very calm and stable around her. I would hate to think what would happen if I were in chemo, and she were to yell at me for the unpleasantness of knowing someone with cancer. I would not put it past her, and I am pretty sure that if you had an illness such as cancer, your friend would somehow make it all about herself.
She certainly is not the brightest light on the Christmas tree, that is for sure. (I had to make a seasonal comment about her, just me showing my holiday spirit

!)
I really cannot tell you how to handle the situation. One thing is for sure, she needs to never disrespect you or the challenges you face in life like this again, if she wants to have an enduring friendship with you.