Upvote:3
If he is caught overstaying in the Schengen Area, he may get into trouble, as in being arrested and deported. Better make plans to leave ASAP.
When he exits the Schengen Area, the border officer may notice that he overstayed. If that is the case, this means that future Schengen visa applications will come up with this information and it is going to be quite difficult for him to get visas for any country in the Schengen Area.
Other than that, Schengen countries will be glad he leaves voluntarily and let him exit.
I don't believe China will know anything about the Schengen overstay, or care in any way about it (unless he gets arrested before he leaves and the Chinese consulate is informed, yet another reason for him to leave ASAP).