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because their stay in China was more than one year, their green card has been void. Unless they have any special reason such as serious illness, they need to abandon their green card and then, apply for B2 visa at the same time or later to reenter US.

Read this website for green card abandonment:
https://china.usembassy-china.org.cn/visas/immigrant-visas/us-citizenship-immigration-services-uscis/abandonment-status-form-407/

Chinese version:
https://china.usembassy-china.org.cn/zh/visas-zh/immigrant-visas-zh/us-citizenship-and-immigration-services-uscis-2/abandonment-of-status-form-i-407/