He's right. The dust cover (the forward part of the frame the encloses the recoil spring) is longer on a Government Model size pistol than a Commander. Of course he could cut down the dust cover to fit, but then when you put a full size slide back on it there is a gap that looks really tacky, even if it might work for a while.
Also, to go from a 45 Norinco to a 40 parts gun, you will have to change the ejctor, which is certainly do-able but not a plug in part (it must be pinned in place). Makes it mostly a gunsmith only project - and another gunsmith project to reverse the process.
Bottom line, you can do this if you're willing to pay for the work, but you won't have a frame that you can easily switch back and forth between the 45 configuration and the 40 caliber commander.
Good luck.