My Ex fucked my best friend who is also my business partner?

This girl who I dated for 4 months.. She was a girl I had been trying to date for years (i really liked her). She suddenly contacted me and we started dating. I had to break up with her because she told me she didn't believe I wanted to be with her even though I tried to show her this wasn't true. In the end I didn't want to hold her with me if she was unsure and unhappy so I had to let her go (obviously I didn't want to do this) . We decided to "stay friends". I saw how she started to get along with my best friend (who is also my business partner) and one night I asked her to please dont proceed with him to anything more. I told my other friends about my fear of her making a move, and somehow she heard about this. She said that because I spoke with my other friends I made her look as a slut (even though my other friends told me that they also saw something was going on). She then again said that since I made her look like a slut then she didn't care anymore about me, and finally, 6 weeks after our break up, she slept with my best friend. :( Now he wants to date her and I dont know what to do since there is also a business involved, which SHE was completely aware of... My friend tried to resist her but she keot throwing herself at him until he couldnt any more. I am more than heartbroken here. I am so lost I feel like a ghost What do I do? I feel I need to end the friendship and the business Do girls that break up friendship regret it?
My Ex fucked my best friend who is also my business partner?
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