Legally speaking, if a person is already married then they are barred from becoming a US Citizen as polygamy is illegal in the United States. Even if your husband divorced one of his wives, his case would still be denied because then he would be trying to get a VISA based off a "fake" marriage AKA getting married/divorced for a VISA which is also not allowed.
Unless the petitioner lies on the application and hides their other marriage, which could result in a permanent VISA ban, they are out of luck.