Can you sue a business for the illegal actions of one of their employees in the course of their work with you?

An employer is liable for the actions of its employees when those actions are done in the course and scope of their employment.  If a McDonald's employees undercooks your hamburger and you get trichinosis, McDonald's is responsible.  If, on a lunch break off McDonald's premises, that same employee breaks your nose in a fight, McDonald's is not responsible.  From what you've described, it seems to me to be a close call whether this person's activities were during the course and in the scope of his/her employment, although if the food service (or someone in authority at the food service) either knew (or should have known) about it, or was sharing in the profits from the wrongdoing, you'd have a much better shot.  Another problem is that a food service rep may not be an employee of the food service; he/she, depending on how much discretion he/she had with regard to how he/she did his/her job, could be considered an independent contractor, for whose actions the food service would probably not be responsible.