What are the requirements needed to get a green card through marriage?

Ok so my cousin's hudsband is an immigrant from el salvador and he was brought here to the US as a child by his parents but they came Illegaly. So now she wants to get him a green card but i told her that she cant because he came here illegaly without a visa but she said that he still could get a green card. Is this true? What are the requirements for a greencard then?