Please help and here is the scenario:
My friend got his green card on April 15, 2005 and then he obtained a position in England and went there starting July 17, 2007. He has visited US at least once every six month after he went to England. Will he have problem applying for US citizenship? If not, does he have to stay in the US for at least three months before applying? Thank you very much and any comments will be appreciated.