Regarding slump (although too late now), if he's really anal about it, he will acquire a K-slump tester. It's a handy little device that you poke into a pile of concrete, and it immediately gives you a slump reading. Uses a calibrated, perforated hollow tube that lets the mortar in the mix flow into the tube. Paid around $25+ for mine 30 years ago (made in Canada, as I recall), so they're probably double that by now.
If he decides to "guesstimate" his slump, placing a shovel-full of mud (on a dry surface) that almost stands up by itself will be good-to-go. Somewhere between 2" and 3".