I can not help you about the anullment issue as that can depend on many things..
As regard to his status.. if he has not yet filed the I485 Adjustment of Status form then he will be required to leave very soon. If they have filed that form then he can get divorced and stay until they are called for the interview, or until the INS finds out.
If he, while an application is pending, divorces and then remarries.. it will look very bad on him. In this event, he will need to see a proper immigration attorney.