Communication with the user is done mostly through the procedure libinter/inter.c. Everything there is procedure table driven, with output for the user being sent back from these procedures in the form of a memory buffer. It should be reasonably simple to take this output and display it in windows, or send it in messages elsewhere.
The only other interaction with the user is in the query gathering procedures that actually get an interactive query from the user. They are in libretrieve/user_q_*.c and would have to be rewritten to suit a different application than the provided "inter" top-level function.
(end of except)