Me vs. His Ex?

Where do I start? I am dating this amazing guy, the only thing is his best friend, is his ex. He was head over heels for her, and vise versa. They are always flirting and I can't stand it. I try and get along with her, considering the fact that they are friends, but.... It's not working. Last frieday he called to see if I wanted to see him before he went to work. I was busy as hell, so I couldn't. He stopped txting me for about 20 min then randomly txtd Jamie scares me. I was like "what?" he went over to her house and let her drive his brand new Mustang when "no one drives it" not even me. I got upset and confronted him and her. I told her to back off him and stop flirting and she flat out sai no. I called and told him to come over after work so we could talk, he said sure. Then he texts and gave me a lame excusse that he couldn't then stopped texing me. Next day I find out he went to her house cuz she called him "upset". What should I do???