父母有绿卡。去年十一月初回中国，原定今年四月回美，后因事情延期到六月初。结果前几天飞来美国的图中，父亲胃部大出血， 飞机迫降日本送医。 两天后情况稳定。现在商量先回中国做具体检查。
--If they come back to US before their stay in China reaches one year (i.e. come back before November), they just need to bring the doctor's/hosiptal diagosis and treatment evidence (e.g. hospitla bills) with them, in case the IO at the port of entry askes them some questions.
If they cannot return before November, then their green card will be void. But your father can apply for SB-1 visa to return to US to reinstate his green card. By the way, SB-1 visa applicaiton is somewhat complicate and it takes a longer time.
Your mother has to return to US before November, othertwise you have to sponsor ger green card application overall again in this case.