I think if someone is actively being hurt in this situation you need to call 911 and get immediate help. Yeah, most situations with police do not end quite well but if it’s serious and looks severe you should record and call for emergency help. I hate calling the cops but I have had to several times and throw away principle if it’s just going south. Most would disagree but drastic measures call for drastic actions
If you can stand to make a scene and get in between a fight, you need to move slowly but intently and keep a close distance between the people and shout loudly but calmly to stop. I don’t think you should even do this unless you are serious about making yourself intimidating and ready to face whatever comes your way. Get the person harmed as far away as you can from the threat.