In my opinion the better way for the manager to handle things would simply to tell the group that he could not accommodate such a large group on such short notice.
It is simply ridiculous to expect a restaurant to go through the trouble and expense of trying to seat such a large group together on short notice. To do so cost the restaurant money. It also inconveniences other customers by making them wait longer while tables sit vacant waiting for enough tables come available for the large group to be seated together.
True. Maybe the manager shouldn’t have taken the group at all.
I might even speculate that if the group hadn’t been black, that’s exactly what he would have done. Wouldn’t surprise me a bit if the manager went against his best interests and took the group in order not to appear racist. That would be the great big ironic cherry on top of this whole thing.