The plumber overtightened the nut. Good thing you didn't put in a fireclay sink or he could have cracked that too. It IS repairable with epoxy by the granite fabricator. But the plumber should pay for it.

I am a fabricator.

Tough problem. If the countertop was properly installed there is no mechanical stress on the granite and no reason for it to crack so the too tight faucet nut is at least possible. I would look along the underside of the countertop to see if it is shimmed so that every part has support. Typically the tops of the cabinetry are not perfectly level and the underside of the stone is also not perfectly flat so some shimming is required. if some section is not supported then a crack developing is quite possible.

However, since granite has a compressive strength similar to concrete it's difficult to imagine tightening a faucet nut enough to harm the granite. In fact, it seems impossible to me.

unless you can determine that the tops were improperly installed your best bet is to let the fabricator repair the crack. A well done repair will make the crack invisible or nearly so.

you might ask the fabricator if the sink area is "rodded". Rodding is the technique used to strengthen the rails around the sink by glueing in a steel or fiberglass rod in the underside of the rail. the rod prevents movement much like rebar in concrete in the event a crack develops. A repair of the crack is much more likely to be permenant if the stone is rodded.