He or She still married but not living together, would it effect the other one status, becoming U.S. citizen?

Season Confalone: Yes it would if you are not divorced legally, it would also be fraud.

Ninfa Asby: as long as you are legally married, for a period of 2 years, you can get your citizenship or green card. the green card is temporary for the first 2 years, after that you can get your citizenship, and a divorce and stay in this country. it is none of there Business if you live with each other. theres a hundred ways to get around that. but you must stay married 2 years.