Going by the variable names they both look like input variables (i.e. input to the DLL function). What does the function do with results?
The two variables will be represented the same way in the Cobol program; you can treat both as just char* and ignore the const. Const is just a contract that means the string won't be modified.
Can you reproduce the problem with a short Cobol program (say 10 lines or so in the PROCEDURE DIVISION)?