is she serious?

One of my best friends, whom I have spent so much money on and time with, has been lying to me for months now. I loved her to death as a friend and I had to find out from someone else she has been dating some boy for 4 months. She also was saying some pretty horrible things to others , like that in fat and I'm a slut which isn't true. I don't know where to go from here. Any advice will be helpful.