As a natural-born American citizen, you may find it difficult to understand why undocumented immigrants do not simply file the papers to become documented. Unfortunately, it is not as easy as you might think. The U.S. issues green cards to a very limited number of people each year from each country. The waiting time to receive documentation is often many years. In addition, these facts are for people attempting to come legally. If a person is already in the U.S. and does not have the proper documentation, there is a whole other set of problems.
Return to Home Country
Whether a person came here without any immigration interviews or documentation or has overstayed a tourist or student Visa, they will need to return to their home country. Once there, they can apply for a new visa and green card. It does not matter if they are married to a U.S. citizen or have lived in the U.S. for many years. They will have to wait until everyone who has already applied has been given their green card or been denied. This in itself can take a few years.
Ban from the U.S.
In addition to having to wait their turn for documentation, they may be banned from entering the U.S. for anywhere from 3 to 10 years. This will depend upon how long they remained in the country undocumented. If they were here more than six months but less than one year, they will be barred for three years. Anything longer than one year will earn them a 10-year ban.
There are a few different waivers available for some undocumented immigrants. If they can prove that being away from a family in the U.S. will cause extreme hardship for them, they may be granted a hardship waiver that will waive the 3 or 10-year ban. However, the waiver must be completed and approved while they are still in the U.S. Once they leave the country it is too late. Other waivers include the DREAM Act which allows people who were brought here undocumented as a child to remain in the country and apply for their green cards.
Unfortunately, once a person is here "illegally" it is difficult for them to become a permanent resident or citizen. There is a very good likelihood they will end up having to go back to their home country for many years. The best way for them to stay here is to talk with an experienced family law and immigration lawyer. There may be a waiver or loophole that will keep them with their family in the U.S for as long as possible.