Why does he need to go to Nepal and marry before his fiance comes to US??????? His filing date and marriage date are not related at all and so the time between those dates is not important. He can file i129f anytime and is not dictated by the marriage date.
He can file the petition now as long as he has met his fiance in last 2 years and bring his fiance here within 5-8 months, get married in the city hall and file GC for her. After she gets GC, they can travel to Nepal whenever they want and they can go for traditional marriage whenever they want or not at all if they don't want that.