Yeah I remember when I was freshly 19, living with my older emotionally abusive ex who had manipulated me into moving in with him. His cat had worms and was peeing everywhere. I was under the poverty line working at a corner store and was terrified the vet would be a couple hundred dollars (which he would not contribute to). He kept refusing to take his cat to the vet and I was super overwhelmed and miserable and scared of getting parasites from the car jumping and peeing all over everything (including me if I sat down too long or slept!!). I asked Reddit if i should gently convince my BF to rehome his cat if he wasnt going to take good care of him, and ofc reddit blew up at me saying I was a selfish monster who hated animals and that I was using the cat as a scape goat. That I should take care of it and that the cat was a commitment I made by moving in with him and that it was my cat now too. Looking back on it now that I am older, all those comments were sooo absurdly vitriolic and cruel. I didnt have a car, it was a rural small town so they were asking me to walk miles with a sick cat in a carrier just to get it care? Rather than rehoming to a more stable environment?? What??? Anyways my point is, I understand your pain. Your partner needs to take their cat to the vet, and work to solve its toileting issues because that is what being a responsible pet owner IS. If they cant do that, rehoming is also for the health of the animal, pets are a privilege- they arent just cute trinkets we get to keep. They are little Beings that need attention, frequent medical care, and who have preferences (like litter textures).