Your client should speak with the employer in the first instance and see whether they are able to issue a W-2C with the SIT refunded.  However, it's most likely too late for the employer to "move" any such SIT now.  Failing which, your client should file an NR return to claim a refund.

I'm always amazed how people could go through the entire year and more not noticing at all that they have wages reported and SIT/LIT withheld to the wrong states!