It could very well void his green card if his green card was issued based upon his being married to you as an American Citizen and you marriage is void based upon the bigamy.  Your marriage is void, not voidable under the law. You can not be married when you get married, as I am sure that you already know.  So what you need to do is to confirm that his first marriage ended properly in divorce.  How you will do this will depend on a few different things.  It may be best for you to consult with ana ttorney on this matter a soon as you possible can.  Good luck to you.