Hi warpig, how are you?
I agree with your sentiments-the tenants are taking the p***...Or poo, if you will...
I can't understand why anyone with a dog would leave poo just sitting there. I pick our dog's poop up straight away when he goes in the yard. What an unpleasant environment it must be for the dog, to have to sidestep its own poo every time it goes out.
Most dog poo is full of "waste", whereas horse and cow poo is mainly organic. I don't think they should have a dog if they can't be bothered to clean up after it. If they disrespect their own environment so much, I can't imagine them bothering to pick up after the dog if they take it for a walk.
I hope that the dog doesn't become ill, but perhaps that the owners do.
As a landlord, I would deem that unacceptable behaviour, and a failure to take care of my property. This would be reason enough to begin a procedure of staged sanctions against tenants. I think though that it would depend on the contract that your friend drew up with the tenants.