The employer cannot continue to employ workers who are illegally inside the US.
In the future, the employer can file for H-2B visas. Petitions should be filed no more than six months before the proposed employment will begin. However, they should be submitted at least 45 days before the employment will begin, because the petition processing and visa issuance may not be completed before work is to begin.
The employer must a prove that there are no unemployed US workers willing or able to do the work. This is established through the state's employment agency using a labor certification process. This process requires a recruitment campaign, including advertising in a local newspaper for available temporary workers.
The duration of the visa is limited to the employer's need for the temporary workers. The maximum authorized period is one year. However, the employer may extend the duration of the visa up to three years -- but with a very close watch from the immigration authorities.
In order to be considered as a nonimmigrant under the above classifications, the prospective employer must file Form I-129, Petition for Nonimmigrant Worker, with the United States Bureau of Citizenship and Immigration Services (USCIS). Once approved, the employer is sent a notice of approval, Form I-797.
It's a start anyway at doing the right thing.