Witnesses

Q: A certain brother wishes to know if his nikah performed in the following manner is valid. In the presence of two jamaat brothers and his wife they first reverted his wife to Islam by making her recite the kalimah tayyiba. Thereafter he told her that I have accepted you as my wife and she also accepted him. There was no formal discussion of mahr. Thereafter they had performed their marriage at the home affairs according to the Christian Law for purposes of citizenship etc. His wife now doubts as to whether they had been living in sin all the time. Please assist.