Staying quiet while someone gets picked on is still choosing a side. Just the easy one.

Neutrality is not actually on the menu in that moment. The person being targeted is reading the room for allies, and silence reads as consent to whoever is doing it. That does not make you a bad person for freezing, because fear of being next is real and the cost of speaking is genuinely yours.

You do not have to stage a confrontation. Changing the subject, standing next to them, or finding them afterwards and saying you saw it all count, and the last one matters more than people realise. Pick whichever you could actually manage, and do that one.