The in-law will be dealing with settling issues for years to come. Loose fill at that depth, will shift, compact and settle indefinitely, especially in areas with heavy rainfall. The correct way to install fill is in layers, typically called "lifts" in these parts. If it was being done by a pro, with at least a backhoe, laborer and compactor, it would typically happen as follows.
After the liner and walls were removed the operator would use the backhoe bucket to thoroughly crumble the concrete floor. Next a lift of fill dirt would evenly loaded and distributed in the bottom of the hole, to a depth of a foot, or so. Fill quality is important. Big rocks or concrete chunks, organic material, or any other trash, will only cause problems down the road. Fill depth would depend on the type of fill material and moisture content. The laborer would then compact the lift using a walk behind compactor, commonly called a jumpin jack. This process would be repeated until the hole is filled to a bit below grade, then topped off with decent topsoil.
Do a job like this by hand, with a wheelbarrow is great, if you are retired, have zero respect for your own time, and have nothing else to do. Otherwise, it can be done by any decent excavating contractor, in a day. Nothing is cheap, but the cost of the job will be highly dependent on the availability, cost and location of the required fill material.
