He/she is a Canadian, period. A Canadian needs a Canadian passport, period.
A Canadian using a PRC passport is probably a crime in Canada already. If he/she uses PRC passport in the process, there could ne a numbers of issues, mail fraud is one of them and any of them can terminate his/her greencard chance. He/she is no longer a PRC citizen.
The right thing to do is to get a Canadian passport.O f course, he/she can always take a chance by doing the wrong thing.