I have found that these types of jobs can lead to better things. If it is a one-time thing, then hopefully you'll be adequately compensated for your work. Walk him through what you are doing and why. Show him what you can recover. He will appreciate what you are able to do and, the next time, he'll have a better idea what makes a good saw log.
I would not, under any circumstances, mill his logs without him being there. That is just a general practice of mine but with low yield logs, he may be surprised by the small amount of lumber, especially if he scaled them himself (not accounting for the loss from sweep or crook), and think that you shorted him.
I now video the majority of my milling appointments. I use a small dash cam and switch Micro SD cards every 3.5 hours. It comes in handy for double-checking my counts, tracking my efficiency, etc.