The bystander has more power than the bully thinks. That’s why bullies hate witnesses who speak.

Cruelty in a group is a performance, and performances need an audience that stays in its seats. One person who does not laugh, who says something, or who simply stands next to the target changes the arithmetic, which is why bullies work hardest on the watchers rather than on the target. You are not a spare part in that situation. You are the part it depends on.

Pick your move in advance, because the moment moves fast. Not laughing is enough. Saying something ordinary to the person afterward is more, and telling an adult who can act is more again.