I can’t see you rinsing with so much water that it flooded the yard with enough water to carry the SH across the lawn to burn it all.
I did some quick calculations to look at how much water it would take to cover a 20x30 rectangle with 3". That sounds like a lot but I was guessing on how much continued flowing water it would take to flow away from the house and saturate the whole area 20’ away. Even if it was half of this, woukdnt the SH be incredibly diluted if you added a couple of gallons to 500+ gallons?
I can see too much fertilizer being applied and burning a lawn.
It sucks when a customer blames you for something you may not have done and you have to appease them to avoid a bad rep. Maybe you should just offer to reseed the lawn and you can spread a $50 bag of grass seed so it will come back with new grass after the winter.
