I'd tell him to fix it. There is no way I'd accept a slab with marks or ridges on it. Grinding the top down will expose the aggregate in the concrete. This will smooth it out, but it won't look right. Grinding it is only part of the fix, and a hardener won't do anything. Either cover it with a high quality epoxy paint to hide the aggregate or seal the ground surface. Sealer can sure be put on after the building is up. Whatever you decide to do, don't pay him for it until you're happy with it. His insurance should cover the costs of him fixing it, not you.

Did you resolve your issue? There are concrete topping products from companies like Ardex that might work in your case.