She can speak to a lawyer in Japan about how to divorce him under Japanese law. If her husband is given proper notice of proceedings that should be valid in the U.S.

She can hire a lawyer in the state where her husband lives and file for divorce there, although she should expect that she will have to travel to the U.S. at least once for divorce proceedings. It may be possible to complete the divorce without her coming back, on the basis of her husband's testimony, but he would have to cooperate (and be trustworthy) for that to happen.