They still may come back to USA with their green card,usually

CBP ( US Customs ) officer still let them enter the country,but detain their green card,then they have to see an immigrtion judge to explain why they stayed so long out of USA,if they can't give a reasonable excuse,their green card will be revoked by judge.
They also may apply a return resident visa in US consulate in Guangzhou,it will take a few months,if the visa be issued,then they may get in with the visa, so don't need to be worry for court.