You have a battery one? Told you yesterday... Well go and get it then. Anyway he started using it and is now strongly considering changing all his tools to Milwaukee based off 5 minutes with that grinder.
I do wonder what some people fill their time with, I often get people marvel at things I have because they didn't know you could get one.
Many old ways are good but people don't seem to be able to differentiate what new stuff is good, I know people who wouldn't buy a diesel because they are slow/noisy and smelly. It's the same with battery grinders, so many people have this belief that they can't do a proper job but if you've got enough batteries and a decent brand there is no issue. If you something like the Dewalt Flexvolt it's a beast
More importantly why is someone who does concrete slabs going to any job without a grinder?