What's wrong with initiating a compost pile...at the bottom of this 8' hole?
Have the friend lay down a thin layer his grass clippings...
Then spread a relatively thin layer of farm soil on top of that...
Again, another layer of grass clippings...
Maybe some horse / cow manure on top of that...
Another thin layer of soil...
Some leaves from the yard, sawdust from the workshop...
Another layer of grass clippings...
Non-meat/dairy food scraps from the house...
Continue this until the 8' hole is filled completely to the brim & then top off with a layer of soil, and mark the original circumference of the hole in some manner.
As thermophilic composting occurs, this in-ground pile will shrink & slowly collapse upon itself like a sink hole.
I imagine in about 2 or so years upon finishing, the entire contents would be ready to be pulled out with a backhoe and be 'harvested'.