I think the point is, should the Chinese authorities find out about his EU citizenship while he is in China, he could be detained for entering the country without a visa and get into major trouble. Having a passport isn't the same as having a citizenship. I don't know how exactly this is handled in China, but if the Chinese citizenship is automatically lost when aquiring another citizenship, it means he was no longer a citizen when entering China - regardless whether his passport was still in his possession.

Of course, if he is lucky, it will probably work. But there is a risk involved.