Shelter Improperly Neutered Cat- Could They Be Liable For The Cost Of Testing?

This happened in wisconsin, to my partner; I'm trying to help him sort it out.

Last summer, my partner adopted a cat from a shelter. I don't know the exact details, but the contract stated the shelter was liable for any medical expenses within the first two weeks of adopting, and costs covering neutering provided that something had initially went wrong.

Cut ahead to now; he's got a noticable ball showing, he's marking, starting to develop tomcat jowls, and humps blankets and his toys really often. My partner contacted the shelter and informed them of the issues, and they stated that unless he had bloodwork done proving the issue, they wouldn't do a thing.

When asked if they would cover the test cost if it turned out he had been improperly fixed, the shelter claimed they wouldn't reimburse him for it. I'm wondering if legally, they could be held liable in the case that it turns out they had initally fixed him incorrectly?

TLDR, could the shelter be held liable for the cost of bloodwork proving that they improperly fixed a cat?