First off I know she can be friends with anyone she wants. Now onto this long story.

So my best friend and I were close friends with this other girl. In the summer of 2017 she got a new neighbor a grade ahead of us. Let's call him C. We all met C and he joined our friend group and at the end of the summer he and our close friend started dating. I never really liked C he gave me a bad vibe but I hung out with him because all my other friends did. 

A couple months later they broke up. And hated eachother and couldnt be around eachother.

 During the break up we weren't friends with C's now ex. Around December we became friends with her again. 

In January it was revealed that I didnt trust C and he said we couldnt be friends which I was okay with. He was still friends with my best friend. A couple weeks later he decided he couldn't be friends with my best friend because she was friends with his ex. He told her this over text in a dramatic "I just cant..." shit and it broke her heart.  A couple weeks later he texted us and begged for our forgiveness which I immediately declined due to him hurting my best friend so badly. She gladly exepted even though I suggested he shouldn't. A month later him and his ex threw allegations at eachother and both of them made her choose a side.
 A couple days later he pulled the same shit as last time and ended it over text with the same dramatic shit pretending he was the victim. Again my best friend was heartbroken. Now to this day it still affects her because she never got closure. We are thinking of seeking closer from him to help her out because she misses him. 

Now she does confront him I would be there with her for emotional support and try to keep herself from forgiving him. Even though she asked me to I know her and she is gonna get extremely mad at me for trying and no matter what I will be there for her. But if I do prevent her from becoming friends with her would I be an asshole because she loves and misses him?

