Pad building

I have an owner that wants to install (build) his own pad for his house slab. It's in North Dakota and it's starting to get cold. He has all the equipment to dig and compact the material. I told him that as long as the material is thawed he was good to go.

The question is, at what point should he stop. When it's frozen 2" deep, 6" deep, 12" deep? I'm not familiar with doing this in the northland. What are some good guidelines for him to go by?