RuthieK, I had a similar experience last year although not work related. I think she had a fit of paranoia and just flipped in all honesty. She had been having treatment for a psychiatric disorder. On the surface, she was wonderful but she took something I'd said the wrong way and really went all out to destroy my credibility. The lies were the worst. She also went to other friends and 'got in first' with a highly exaggerated story. As I said, the lies were the worst. In essence, she couldn't do me much damage but like you, I felt bereft because I really valued her as a friend.
But you do learn and looking back, I think I had a lucky escape. It's better to have friends outside of work and you need to choose carefully. I have one good friend that I've known for 25 years. I rarely see her but when I do, it's as though we see each other all the time. I have lots of other friends but I am careful with how much I say.
You will get over your lost friend, time is good healer, it's true. Strike out in a new direction, start being independent and if you're like me, and have a good husband, make him your new best friend.
Get a dog and a cat too. They never let you down.
Chin up! Don't dwell on what's gone. Map out a new race track and get on it. xxxx