Therefore, I believe that a person who witnesses another wrong does not have an obligation to help, but it should be highly encouraged. We never know the situation of the bystander---some may have their lives jeopardized, other may have trauma that prevents them from acting out, but at the end of the day, helping out is a choice, not a law. There should not be any legal obligation for witnesses to help out in difficult situations.
Much of the bystander energy also stems from societal norms and societal issues that has more to do with society than the individual themselves. In Judy Harris's "The Bystander Effect in the Cellphone Age," watchers of a fire decided to record fires blazing on a 3-story building rather than check to see if anybody was still in the building. Passengers on the 36 bus in a Boston Globe article by Brian McGrory ignored the assault of an 8 year-old boy by a much older man. We cannot always blame the witnesses, but instead the system in which society raises them to ignore. For example, our phones have been an easy way for us to escape unwanted situations, and can sometimes even act as a distraction. Now that we're becoming more technology-driven, we're relying on these devices more to help us cope with stress. In America, we also have a strong mind-your-own-business vibe, and it's really not easy to break out of routine to help someone.
Lastly, I agree with @deviouseggplant24 in that I think that I would be the person standing up in a situation like this, but it's incredibly scary to witness and I'm not too sure I would have acted heroically and stood up. Formerly, I've always been afraid to challenge anybody, even the people I was close to, in fear of rejection. I hope my post reveals a couple insights into the possible arguments in defense of the witness.