Can a non-citizen foreign national divorce his pregnant US citizen spouse and remarry?
Yes. The foreign national would not yet be eligible to apply for permanent residency therefore divorcing his or her current spouse and remarrying would not affect his or her status. Be advised, that it may create difficulty when the application for permanent residency is made.
The U.S. citizen files divorce proceedings in the U.S. county where he or she lives and the fact that the defendant is a Mexican citizen does not prevent the plaintiff from obtaining a judgment of divorce. If the Mexican spouse is not physically present in the United States, that will make it more difficult to serve necessary court papers, but not impossible.