You have two options. The first is to contact the Dept of Labor & Industries. This process is slow and many clients have complained to me that it is ineffective. It has the advantage of being free of charge.
The second option is to have an attorney write a demand letter for you, pointing out that the employer will be liable for double damages and attorney's fees if you have to take legal action. This usually shakes the money loose unless the employer simply has no money.
You could also try letting the employer know about the liability for double damages yourself, but unfortunately, most employers who have the inclination to withhold wages unlawfully also are unresponsive to such feedback from former employees.
You do not mention it in your post, but your fiancee may also want to look into the grounds for firing and determine whether he qualifies for unemployment. Many employers will label as a discharge what is actually a layoff, hoping to avoid an increase in employment security premiums.