Legal Question in International Law in California

I presume his U.S. passport is expired by now, so maybe a good place to start would be for him to apply for a renewal passport and see if he encounters any problems. If he gets back a passport that says, "Only valid when used to return to the U.S.," then he has a problem.

This question is a bit confusing. A U.S. citizen can always return, though, as the other respondent indicates, he'll need to apply for a new passport through a U.S. consulate. Second, it isn't clear whether he was forging a U.S. passport or some other country's passport. If it was a U.S. passport, it would violate U.S. law, with jurisdiction in U.S. courts. If the charge was dismissed, then it had to be in a U.S. court (or possibly a U.S. attorney declined to prosecute). If the forgery was of a non-U.S. passport, then the U.S. does not have jurisdiction. (Given the passage of 10 years, this may beyond the applicable statute of limitations.)