and Xiao also returned to China so most likely
ababndon his permanent residency in USA
(unless he applied for a re-entry permit which
allows applicant to stay outside USA for maximum 2 years).

If he is nether a US citizen nor a US resident,
I doubt he can file a lawsuit in a US court.
This is matter of jurisdiction.

If he insists upon his permanent residency in USA,
someone should investigae whether he has
done all the legal requirements to fulfill
residece requirements, which include many things
include taxes.