8/2 wire for 240v EV charger
Yes, a 10 AWG ground wire is fine for up to a 60A circuit.  

The 8/2 Romex (NM) cable is good for amperage up to 40 amps. However, the breaker must match the socket / EV charger spec.  If the socket is a NEMA 6-30 or 14-30, the breaker must be 30A.  If the EV charger docs specify 30A breaker, then that must be so.  

Again, the 8/2 cable is perfectly fine for 30A or really any size up to 40A.  So if your spec calls for a 30A breaker, you only need to change the breaker. 30A sockets will accept #8 wire. 

It is possible that a 30A-nameplate charger might want a 40A breaker. The documentation will tell the tale.