Hi bjquat,
to All Things Plants from me too!
How large is the area of the indentation in your dad's lawn? Have you checked to see if the ground is wet in that area? I wonder if perhaps their was possibly a break in an irrigation line in that part of the yard, causing erosion of the soil and sinking of the lawn? Or, perhaps there are insects of some sort beneath the lawn, eating the roots of the grass and causing the sinking appearance? I hope you are able to get another landscaping or irrigation company to come out and check the area. Hopefully it's something minor that won't be a big expense to repair.
