friendship/relationship

So i’m 14 and in 8th grade. My best friend is truly amazing i’ve never met someone like her until now. But recently, sarah (i’ll calm her sarah) met and started to date boy (we’ll call him james) Sarah just started at our school and didn’t really know James past. James went to school with me last year and he is now a freshman. When I went to school with James, he would punch holes in the wall and smash the glass in doorways. So to me he already doesn’t have a good profile. So, they seemed good for a while until Sarah would tell me some weird things. James would tell her he would “kill him self” if she didn’t talk to him or hang out. This really scared her because she takes everything to heart. He also had some more stranger activity. He smashed his phone, when we were all hanging together he showed us videos of people dying and he was laughing the whole time, he would say he wants to die in the military, and then he would punch stuff. When i’m with Sarah and she facetimes him he would punch stuff and he basically broke his hand from doing it so hard. So basically his behavior makes me fear for my friend. But he broke my last straw when a few weeks ago, Sarah called me crying as she had locked herself in the bathroom because he was scaring her. I explained to her that your boyfriend shouldn’t scare you like that. (i have confronted her many times but she always tries to change subject) Later that night her mom texted me telling me to try and get her to dump him because she didn’t like him and made her feel uncomfortable. She then informed me that James shoved Sarah when she was trying to get him to stop punching himself. They are now not allowed to see each other and they were supposed to “break up” but they didn’t. Recently a friend told me that Sarah was planning on meeting up with James and not telling her mom. I told my mother this as I didn’t know what to do, my mom says that it’s not my circus and that she can make her own decisions. A part of me is really done with all of this because she always ignores me and it’s no use but if she got hurt I couldn’t live with myself. What do I do?