In the heat of the moment, it can be hard to reason with someone who is bullying. Only when and if it feels safe, find an opportunity to talk with the person afterward, when they may be more approachable and less influenced by the presence of onlookers or peer pressure.
People who bully others sometimes feel like they have a reputation to protect. You might have a better chance reasoning with them in private instead of in front of a group.
It’s okay if you regret not speaking up in the moment. Only when and if it feels safe, try approaching the person who bullied later and explaining how what they did was hurtful. They may not even realize that what they did was wrong.
Someone might act like a bully in certain places, such as at school or in front of friends, but actually be a nice person in other places. If you wait and approach them in another context, it may be a great opportunity to talk about the issue.
Consider phrases like, “Hey, can I talk to you about something?” “I saw what happened before. Do you think we can talk about it?” “How would you feel if somebody said that about you?” or “I’m getting worried about ___. Maybe we should leave her alone.”