我一朋友持有绿卡，因为褪摔断有4年没有来美国，现在褪好了想来，不知入境会否有麻烦，他应该以何种方式才可以来美国。
-He will not be allowed to enter US using this green card. He needs to apply for the Returning Resident Visa (SB-1) visa in anyone of the US Consulates in China and then, use the SB-1 visa to enter US to activate his green card. If he can not get the SB-1 visa, his green card is void.
For SB-1 visa, see this website:
http://travel.state.gov/visa/immigrants/info/info_1333.html
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