Understand the differences but since his main footage is DVCProHD with all its limitations I would keep it all the same, his HDV footage is not 10 bit anyway so once again there is no real gain in going to ProRes, the banding and jaggies are already in his footage.
Now if you have 10-bit material to begin with I probably would not go down to DVCProHD because of the reason you listed, but that is not the case.
But in the end it is not that big of a deal, main thing is to get it out of the HDV codec.