-The mother's green card has been void and thus, she cannot use it to travel to US. If the mother had serious illness which prevented her from returning to US on time, she can apply for SB-1 visa (Returning Resident Visa) to enter US to reinstate her green card, but she needs to provide the strong evidences of the medical treatments, doctor/hospital diagnosis, etc. to prove that she cannot travel during the period of time. SB-1 is very strict, so she needs to prepare for the application.
Read thess links for SB-1 visa application procedure:
https://travel.state.gov/content/travel/en/us-visas/immigrate/returning-resident.html
http://www.ustraveldocs.com/cn/cn-iv-sb1visas.asp
Another choice is that she may just abandon the green card in the US Consulate, then apply for B2 visa to enter US. Then, if she wants to get the green card again, your friend can sponsor her all over all again.
Read this link for green card abandonment in China:
https://china.usembassy-china.org.cn/visas/immigrant-visas/us-citizenship-immigration-services-uscis/abandonment-status-form-407/