Homeowners insurance most certainly doesn't cover flooding of homes. That's why people in flood prone area's purchase special flood insurance if available. Sometimes its not available if the area is too high a risk. Also, if it does flood and his neighbors are affected and don't have flood insurance, and the flood is a direct result of Greg removing vegetation & soil, he'd be super fucked.
Homeowners also doesn't usually cover intentional acts of damage. Him not knowing this was damaging to the property doesn't exclude him from this, they still won't cover anything the owner did intentionally - and he intended to remove the stuff to make a bigger space in the backyard.
Just checked out a few sources about homeowners insurance and the land/property the house is built on. No surprise, it's a no go:
"Homeowner's insurance generally does not cover damage to land, landscaping or concrete, but getting land coverage is often not necessary."
I'd be shocked if he purchased any type of property insurance or land coverage, so he's most likely screwed.
I don't know what the price of his house was, but I saw the info on the mortgage dated around March 2018 and the loan amount was around 450k. They didn't put down a lot of money at all. Im sure JG Wentworth will be thrilled to hear about this.
ETA: What's Plainey's take on all this, does anyone know? She can't bury her head in the sand, she's on the deed and the mortgage so she's liable for all this as well. Even if they were to declare bankruptcy in an attempt to not pay it won't discharge any possible debt that can come from this. It would be wise for her to seek out an attorney to help them navigate this even if they think it's not worth it. In the end, they could very well end up in a shit ton of debt with a tanked credit score and liens up the ass.