Greg - tough position you are in with your client. But I do have to say from the owner's side that I would feel that was our position to catch, not the client's. I would be extremely disappointed in my staff if they hadn't checked the clicks prior to running the job. Contrary to Che.c's attitude of "screw the customer - they set it up wrong", which will drive your business to extinction, the customer thought it was black, and they shouldn't have to investigate and learn all the in's and out's of our business to get a print job run. I see professinal designers with years of experience making these same mistakes. It is simply ignorance and a misunderstanding.
We use a Fiery, and struggle with this from time to time. If the file is set properly, the Fiery will recognize the monochrome pages and click accordingly. Sometimes, we'll rip over a few select pages seperately at grayscale to the mailbox, and then re-combine the document on the machine itself. Or we'll use PitStop, which doesn't always work as people claim. Whether the global change, or selective editing, it can still sometimes vex us. Even doing a 'select all' on the page and converting, it can sometimes still click color and we bang our heads trying to correct. As Ken said, sometimes a little thing like a crop will hang you up, or an unseen variable off the page. It's a bitch and can be a time vacuum trying to correct (which should be billable time), but it can be done and should be caught BEFORE the job is run, not after.
If I can diverge a bit, Ken, what is your process in PitStop? And how has it worked for you (always/sometimes/depends, etc.)?