Is there specific code on the books in Georgia for or against an employer providing a negative reference against former employees? Or anything about it that violates federal law?

Thanks

No and no. In fact the reverse is true. The majority of States have passed "hold harmless" referencing laws that protects an employer from liability when they give a reference in good faith.

In any State, an employer is free to provide reference information. As long as the employer is providing information that is factually true or represents their honest opinion, they are free to do so. That includes telling other employers about an employee's poor job performance and/or reason they were fired.