It is nearly impossisble for them to get the SB-1 visa. My suggestion: they had better abandon their green card, then apply for B2 visa to visit you. It may take a few months for green card abandonment process.
See this link for green card abandonment for Chinese:
Abandonment of Status (Form I-407) - U.S. Embassy & Consulates in China (usembassy-china.org.cn)
选择“Disable on www.wenxuecity.com”
选择“don't run on pages on this domain”
