As long as he meets the residency requirements of his current state and county, he can file for divorce in the U.S. -- the U.S. court won't be able to resolve custody and property issues for a relationship and assets that exist(ed) solely in Mexico, if there are any, but it can grant him a basic divorce.
The court will require that he make a good faith effort to serve his wife with notice of the divorce, although if she cannot be located for service the court should allow substituted service (public notices, publication and the like) and allow the divorce to proceed once its order is carried out.

