Marry an American just for entry to the US?

My cousin married a girl in China and brought her to the US. They met on the Internet. They "fell in love" (through an interpreter (she speaks no English at all)) and he got a pport and took his very first flight ever --- to China. He wore his wedding suit on the plane because he didn't know any better, spent a month with her, and they got married. He came back, re-mortgaged his house and sold his car and borrowed money to bring her to the US. We all thought he was going to go bankrupt! She seemed nice enough when she arrived, but I knew my poor, innocent (naive) cousin had been duped. Three or four months or so after she arrived, they went down to Charlotte to get her green card. Within 5 days, she'd left ("for New York"). Now she's come back demanding a divorce AND she wants alimony. Isn't this kind of thing illegal? What agency does my cousin complain to? It's one thing for her to use him -- but to take his last dime thru alimony seems unbelievably heartless. Help!!!