I am so sorry to hear this. As I said, I am new to this landlord business. I would think that something can be done. Has your daughter and son in-law tried contacting the city? If the home is in Granite, doesn't granite have some sort of regulations in regards to crap being in the yard? I am not 100% sure as I haven't live in Granite in over 2 years, but I would think so. I know they have regulations about the grass being too high. They will send out a notice, and I believe a fine.
I would highly suggest contacting the city, and if need be taking them to court. While going through the courts you can take the rent money, and hand it over to a lawyer each month, rather than giving it to the landlord. It is fully legal.
Wish I had better advice for you. I do have a home coming open soon if your daughter would be interested?! lol I feel I am a great landlord. I truly love my renters and am sad to see them go, but understand why they must move. We have maintained the home as needed. It is also renting for $850. Which I feel is a great price given the home, and the neighborhood it is located in.