You could ask some of the Ayn Rand fans around here. They would contend that none of the witnesses or bystanders have any moral duty to the victim, but it would have been ok for them to get involved if it served their self-interest.
Strawman. You don’t understand objectivism. Nothing in it says you cannot help others out of empathy, fellow feeling or just a sense of justice. You just cannot be used by others as a bandage for their wounds.
I am pretty much a Randite, and I have intervened in crap like this more than once because I hate thugs and don’t want them to feel like they can do this stuff without consequences. However, in California, where I am forbidden the means to defend myself, I might not.