Can a Christian woman Marry a Muslim man legally in Pakistan?

I have a friend who is Christian, she is a US Citizen and has a very good job. She left to Pakistan 3 years ago and met a muslim man thru a mutual friend. She only knew him for 3 weeks before she decided to marry him. They got married on a piece of paper. After she came back to the US, she sent paper work to the US Emby in Pakistan to sponsor her new husband, but it has taken 3 years and finally she was scrutinized by immigration officials about her legality of her marriage.