Calling out a racist joke in the group, even calmly, is a risk worth taking every single time.

What makes a joke like that work is the laugh, and silence reads as agreement in the moment even when it is only discomfort. Saying something breaks the assumption that everyone present is fine with it. You risk being called humourless, which is a small price next to being counted among the people who were fine with it. Calm matters here, because flat and unimpressed is much harder to deflect than anger.

Have a short line ready, because the moment moves quickly. Not funny, or I am not into that, said plainly, and then carry on with the conversation.