Wfp doesn't leave smears. Tell him to clean his windows on the inside, or ask to do them for him so he can properly see the difference.
I had a customer say that, I said, please show me the offending window, sure enough lots of smears, but then on the inside, got my mop and squeegee out, cleaned it. Customer embarrassed. Simple as that.
Training your customers is the most important thing about wfp.