How do you become a US Citizen?

I think there is no problem when you have the marriage certificate as a proof to apply for citizen of the intended country. But again this needs someone from US to give a vivid answer.

Your in luck, this is probably the easiest visa to get actually. Her fiance will need what is called a "k1 visa". Im not sure about the aplication process, but it is typicly issued and you must be married with in 90 days of the visa. There are a few interviews and a follow up to mkae sire it wasn't just a marriage of convenience and such though. I would just go to the U.S. customas and immigration website and read up on it, then make a few calls and talk directly to them if you still have questions. They are the best ones to get direct answers from.