just remembered something.... if you wife's PRC passport was renewed after she got her U.S. greencard and she hasn't exited/re-entered the US since, it's a good idea to make sure the US CBP stamps her PRC passport upon return from Grand Cayman. (especially if you live in a consular district that's under jurisdiction of the Chinese consulates in NY and Chicago... not sure about others, since those were the only two I dealt with on behalf of my wife)

Of course, this will no longer be an issue once your wife becomes a naturalized U.S. citizen because she will automatically lose her Chinese citizenship and the PRC passport will become invalid.