What exactly can a previous employer legally tell a prospective employer?
An employer is free to furnish information about an ex-employee as long as it isn't given maliciously or with the intent to harm the ex-employee. Employers cannot give out confidential or private data, such as medical records, and as long as any information given is for a proper purpose, the employer is protected from being sued for defamation. The information provided must be only true factual statements about the employee's performance.
Laws vary by state.
However, an employer could be held legally liable if any false information was provided, especially if it is disparaging or accuses an employee of criminal behavior or immoral conduct, unless the employer believed in good faith that the information was true.