I would guess the cat (they can soil on things when stressed, or if they're locked away from the litter pan) Did it have loose stools in the litterpan?
That said, no one is going to be able to prove without a doubt which animal did it. Throwing that much of a fit over some bedding is just ridiculous. Bedding washes. I hope this person never has children, or they're going to be replacing a lot of blankets/clothing if they discard something that gets soiled on
I have an elderly dog with cognitive issues who sometimes soils things, yet can't be confined due to his cognitive issues. We clean up and move on. And, as a vet tech, I can't tell you how many people's pets pee or poo on me. Or, even better, anal. I wash up, wash the clothing, and go about my day.
And, what sort of bedspread costs several hundred dollars?!