When the room is laughing at somebody, one person saying “that’s not funny” changes the whole temperature.

Laughter in a group creates the impression of agreement, and that impression is what makes the person at the centre of it feel surrounded. One voice declining to join changes the reading of the whole room, even if nobody else says anything. It is a small sentence with a disproportionate effect.

If it happens in front of you, say it plainly and without a lecture attached. Then move the conversation on rather than making a stand of it. Check in with the person afterwards, away from everyone.