How long would they have to go back to the country as the person has nothing left in that country? Also, would they get into difficulties when they leave America?

Hello, I have a friend (U.K Citizen) who has fallen in love with a person that lives in the U.S but that person that lives in the U.S is illegally living there. They are both engaged and found a way to get married person but they don't know how the person living in the U.S illegally will be able to ...