Fiancé and his friend

My fiancé of 3 years had a best friend for 4 years who was against me the entire time we were together. She was never respectful of our relationship and I saw texts from her talking trash on me to him. I found out they started talking again. He wants to be friends and go out alone with her etc. I said I want her to be nice to me first and offered going on a group date but he refuses to even try that. Am I wrong? It's so hard for me to even be ok with them being friends! I'm trying to become her friend so I can feel more comfortable with them hanging out and know she can respect us.