It was version 5, purchased for them in '96 - '97.
I know zip about DBF files but I do know that some of the underlying standards have changed over the years - as is natural.
The aspects of Monarch export to dbf that might come into play are that there are 4 export files types for dbf files and the MS Jet 4.0 export engine is in the mix somewhere.
I assume that the target application is expecting either integer fields decimal with the number of decimal places defined by the incoming data and limited to 16 chars? (Or something like that ...)
In which case you may have been using the Monarch V3 export type with the Monarch V5 system.
Have you tried setting the export file type to dbase III (Monarch V3) ? If so what was the result?
The other thing to check is that the client has suitable versions of Jet installed. It might be worth checking the Microsoft support site for information and to check for potentially useful updates just in case that would help.
I hope this helps you to move forward towards to a solution.