Quick background - cause there is alot unknown.
Agribusiness and meat packing in Iowa has a long and checkered history of using undocumented foreign nationals. A book was written about a very ugly episode: Postville: A Clash of Cultures in Heartland America
That was a case of a meatpacking plant. Terrible results. Also a documentary.
Agribusiness uses 'seasonal workers' (I believe that is H-2B visa). Agribusiness also uses other foreign nationals. There have been cohorts of people from Bosnia, Mexico, Central America (the USA has a history of this). There are always shenanigans often with the owners.
Employers get in trouble all the time for violations. Some meat packing plants go out of business. Other 'farmers' (really agri-business) are prosecuted. (you can google it). One agribusiness got in huge trouble for basically enslaving developmentally disabled adults. The workers get into trouble too.
Construction also uses H-2B visas, but that makes sense in that things are seasonal. There are not enough 'citizens' to meet employment needs.
So a person could be a naturalized citizen, a citizen on a visa (including an H-2B), or an undocumented foreign natural. Businesses are supposed to be tightly regulated, but as you can see that is problematic -- people cheat, owners cheat etc. etc.
I would be careful about labeling this person 'undocumented'. Who knows. The farm where he works belongs to a prominent and well connected family. I would not want to be in their shoes.
Hope that is some background worth considering. And not to make excuses for bad behaviors on the part of employees.