If the bully is your friend, standing up costs more. That’s exactly why it matters more.

When the person behaving badly is in your own group, you lose the easy version where you are an outsider objecting. Now you risk the lunch table, the group chat, the ride home. But your silence also carries more weight, because a friend going along with it is the permission the behaviour runs on. Withdrawing that permission is the one thing an outsider cannot do.

Say it to them privately first, since people defend themselves harder with an audience. Name the specific thing and what you want stopped. If it continues, stop laughing, stop joining in, and be visibly done with that part of them.