No. Their green cards are void.
They may go to the US Consulate in Guangzhou to apply for the SB-1 visa to enter US to reinstate their if they can convince the Visa officer that their overstay in China is due to their illness. But the SB-1 visa is very strict, so they have to prepare all the documents/evidences.
Another choice is they may just give up the green card in the US Consulate in China, and then apply for the B2 visa to enter US. Then, apply for the green card again.