First of all, understand that your fiancee won't be working there much longer if she makes a stand. She doesn't need to be working in such an abusive environment, anyway, but she does need to anticipate the financial impact of losing her job.
That being said, I'd report this employer to your local office of the US Department of Labor, Employment Standards Division and Wages and Hours Division, to the nearest field office of the IRS, and to your state's employment security agency. The unsanitary conditions may be reported to your local and/or state departments of health, and the undocumented worker situation can be reported to the nearest office of the US Department of Immigration and Customs Enforcement.
Of course, it goes without saying that you need to have good, hard evidence to support your accusations, or you might find yourself liable for slander or defamation.
1. Your fiance needs to become an undercover sleuth - and keep QUIET about it! Do everything she can to get info on those illegal workers, names, where they live/stay, etc. Record dates & times & events of abuse, etc, observed. Record exact hours worked, doing what, and keep all paystubs, etc, payment records given by the employer. In other words, she needs evidence of absolutely everthing going on there.
* * * She needs to make her notes after every shift after she is off the clock and off the premises! It is imperative that she only appears to be a diligent employee, and not "too nosy" or "sleuthing." * * *
2. Call ICE Hotline to report the illegal aliens and everything going on with them.
3. Next, call IRS Tax Fraud Hotline to report all tax violations, "cooking the books," paying employees under the table for part of hours worked, tax fraud. Include also if they are claiming to pay (or withheld from your pay) Social Security & Medicare taxes, but those amounts are not showing up on your annual contributions report from Social Security Administration.
4. Then, if they are messing with Social Security contribs, call Social Security Administration fraud hotline.
5. Next, call state income tax and sales tax authorities if they are screwing around with these!
6. Last, call your state's Department of Labor Wage & Hour Division to report they hire illegal aliens & don't pay them (except provide housing, room & board), fail to record full hours worked, pay "under the table" for some hours worked, fail to provide breaks as required by state laws, abuse workers, etc.
In each case, get a name & fax number of someone at the Fraud Hotline to fax copies of relevant evidence gathered, her daily time-dated shift notes, etc. Take these sobs out! Nobody should have to put up with this. She needs to find another job as soon as she has evidence required and makes the various reports to authorities, unless they specifically ask her to remain to gather more evidence.