If the employee cashed the checks it is considered theft, and depending upon the total amount it could be a felony. The company can file criminal theft charges with the D.A.
If you are the employee then contact the company asap to see about making payments to repay it. Some companies will not make payment arrangements some will, they do not have to. Also, make sure that they contact the IRS to let them know you did not earn that money so it won't be a tax liability next year. Good luck, Susu

