I think a Chinese visa is needed under the current rules. They check your id when you exit. If you do not exit with the id you came in with, you will be in big trouble.

The only way to get around this is to add a third country. (i.e. enter JP as EU, enter China as CN, leave China as CN and leave JP as EU, you will also need a JP visa on your Chinese passport)

BTW, he can probably only keep his Chinese passport for 5 yrs max, unless he keeps his Chinese id and renew his passport in China.