Often, it is more productive to use what one already knows.

FWIW, the people - including software developers - for this particular client of mine are especially adverse to the command line. With other clients, it has been enough to make sure to wait for input (on STDIN) when the (other command line oriented) program is started without parameters or options (as happens when a user double-clicks on the file). And, of course, state in the shown help text that the program needs to run from the command line. Since the programs I'm trying "friendly up" just read 1 or 2 files and write another, the only GUI needed is file open/save dialogs.