I wasn't surprised to see you say "he". This problem seems to arise from men much more often than women - I assume it's something to do with small penis size.
Of course you're right, the resolution number is completely irrelevant. But educating idiots about this is really difficult. This is the best I have, and I realise it's feeble, and not targeted for this case.
In short, what he needs to understand is that you can change the files to 300ppi for him if he insists on it, but the files he receives will be EXACTLY the same number of pixels, and therefore will be printable to EXACTLY the same size; and you will charge him a fee for making this pointless change.
Or, you can just change them. It's simply a matter of opening the Image Size dialog, making sure the "Resample Image" box isn't checked, and changing the resolution to 300ppi. You can save this little process as an action, and run it through Image Processor.