It seems fairly clearcut.
I'm going to leave out diversity - how he have filled out the diversity entry is probably his best way forward (and divorce the other one).
Polygamy is legal for Muslim men in most Muslim countries.
If the husband is a US dual citizen, then he broke the law. Pick one wife, divorce the other. It is arguable whether the non US wife broke any laws here.
So ASSUMING US wife is sponsoring him, then he has to go with that US wife and divorce the other. The US wife (however) broke US laws by practicing
polygamy - so entirely possible embassy will never give him an immigrant visa.
ASSUMING someone else is sponsoring him, he WASN'T committing a crime by practicing polygamy. Now that he desires a US green card, he must
divorce one of the wives. That wife gets to immigrate with him.
He will have GREAT difficulty bringing the other wife even if he divorces 1st wife and sponsors the other.
Someone who is of the mindset that polygamy is OK (given the great SOCIAL costs of legal polygamy in Muslim countries) should maybe reconsider his decision to immigrate.