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Was the customer wearing a mask of the style the violent invaders use to hide their faces from legal accountability? If so, then yes it sounds like the restaurant owner should have suspected they were one of the gang and not served them.

If they removed their mask before getting to the restaurant, and the restaurant owner had no other reason to suspect them, then the restaurant owner is in the clear. But hopefully someone took a picture of their face so they can be on the early admission list for Nuremberg 2.