Hey nepwanted, i heard for GC holder wanting to stay outisde of continental US for more than a year can pay some sort of $100 fee?fine? per year and live outside upto the life of GC. i haven't verified it but heard it from a creditable source, so you might want to check out uscis.gov or may be even send them an email. if nothing works, just have your folks make a new nepali passport before they re-enter US, as the new passport won't have any departure date and immigration doesn't keep track of when you left the country other than from what is stamped on your passport. plus they don't really profile elders that much anyways so i wouldn't be too worried...my folks haven't had any issues staying pass a year on many occasion.