The very first step in Amazon Brand Registry is to tell them your US Trademark registration number. Without it, I'm unable to get through the Brand Registry website.
And I can't find any email address for them - nor, any place to mention this to. That's why I posted that to my website's home page.
Total speculation here, but probably because common law trademark would have to be proven in a court of law. That is, he would have to sue the Chinese company, present evidence like newspaper clippings or testimony from customers, win, get a judgement, and present that to Amazon. Until then, Amazon looks in the USPTO database.
Since he has common law trademark, why wouldn't that still apply? Someone else is selling via Amazon in the US using his common law trademark.