Sounds to me like he should not be installing the crown until the heat is on in the new house and the molding has been sitting there for at least a week for it to acclimate itself to the temperature and humidity conditions that it will actually be "living" in.
Another suggestion is to take a 2x4 and rip it to the angle of the crown and a little smaller than the cavity formed by the crown and the wall and the ceiling. You can use larger nails to put the 2x4 into the wall and ceiling junction. Obviously, you can only nail the crown every 16" though. But when you install the crown, you can nail anywhere. Using the 2x4 nailer makes it easier for one person to install it also.