In this case, I would try to reach higher managers to ask them for confirmation of the reason why my friend was not hired despite the fact that he was a perfect fit. I would also ask to reconsider him or conduct one more interview with him. If this would not work, I will inform my friend about the decision of the HR manager and the reason of denial. According to EEOC (2015), my employer has no right to punish me for the complaint about discriminatory policies so it would be fair to tell the truth.
Further actions depend on the friend. He can address the U.S. Equal Employment Opportunity Commission in order to deal with this situation effectively. Alternatively, he can search for other job where his accent is not treated as something that would make his work less valuable for the organization. Anyway, I would try to reach higher standing people inside the organization before telling my friend about this unfair