Speaking up for a coworker who’s being treated badly might cost you a favor. It’s worth more than one.

Workplaces run on small favours, and standing up for somebody can spend one of yours, which is why people stay quiet and call it staying out of it. What you buy is less visible. The person being treated badly knows, and so does everybody else who watched it happen, and that kind of reputation outlasts any favour you were saving. Nobody remembers who kept their head down.

You do not have to confront anyone publicly. Say something kind to the person afterward, and one neutral sentence to whoever is doing it while other people can hear.