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Mail the I-94 to the following address along with proof of U.S. departure such as the Entry Stamp to China on the passport by the Chinese Customs when they entered China after they left US (they need to copy all passport pages that are not completely blank, and include the biographical page containing the photograph), the airplane boarding card, etc., and explain in a letter that they left US on what date and forgot to turn in the I-94):

DHS - CBP SBU

1084 South Laurel Road

London, KY 40744, USA



Here is a website for your information:
http://www.immihelp.com/visas/returning-uncollected-i94-form-to-cbp.html