What should I do, help!?

My best friend dated this guy and he was pretty nice but then he dumped her! So after a while they went back out again and he dumped her again. And they went back out again and they said they loved each other but then got in a huge fight because he wanted to be her main priority but she didn't! So she broke p with him! and he said he would never go out with anyone again but a week later her went out with a girl named lizzy for two days! And then broke up with her because he felt bad !Now with all this in the past it has been about a month or two, we went on a field trip to the imax and my bff aka molly sat by a friend that just happened to be a guy and brier aka the old bf got all pissy and texted her and was like why did you sit by trey do you like him are you going out ? but it was none of his business and she said sorry for doing nothing wrong! What is your guys opinion? Do you think he was right or she was ? Should I intervene or what?