I agree that the only way the disparate treatment would be illegal would be if it was specifically because of the ethnicity issue. If the employees are represented by a union, the suspension can be challenged on the basis of unequal discipline regardless of any ethnicity based discrimination. Without a union, however, the employer generally has the right to be unfair.
If the basis for the unequal discipline is the employees' ethnicity, that likely would be unlawful discrimination.
However, just because the employees are of different ethnicity does not mean that was the reason for the different discipline. There are many lawful reasons why an employer treats employees differently. Perhaps one employee has a more credible story than the other employee. Perhaps the disciplined employee already had a history of being disciplined. Perhaps the employer, even if factually mistaken, thinks the disciplined employee started the argument. Perhaps the undisciplined employee is a personal friend of the employer/supervisor.
More information about unlawful discrimination is at www.hum.wa.gov .
You can review your specific facts with your attorney to find out your legal options.