Message codes should work as long as they exist in your ListMail installation as global or list-specific message codes.. There is no reason why these codes should stop working unless the code data has been removed or was, perhaps, temporarily inaccessible.
Please note that I do not allow re-distribution of my program to people who have not purchased through me (or who you have not compensated me $50, as per my affiliate agreement, for an official owner/membership of Listmail)
Your client should personally post any further concerns here for me to answer.