what do I do?

Okay so my childhood friend who I don't really talk to that much anymore is starting a fight with my boyfriend and it is scheduled to happen this Monday coming. I don't want him to start on my boyfriend as I well and truly think he is the one and I don't want to lose him as my "childhood friend" has threatened to kill him. What do I do? I can't stop the fight because my boyfriend won't let me and I am scared that if I go to support him I am going to break down from seeing him get hurt.