Just take my advice carefully ' police station and her father in law not there job to prove it , all you need paper from (Kảmnạn) or (phu yai ban) under Kảmnạn one of them enough to make it for you and that would be enough approved you are leaving there 100%
Ask your wife to take police paper and her father iin law and if someone around her know him personally in case bring 2 witnesses to Kảmnạn would be a piece of cake to make it and that his job