Dog kidneys are kind of in an awkward position inside the body. Unlike our own (close to the back, under the ribs), a dogs kidneys are more towards the middle of the body, if that makes sense. The likelihood of Kero damaging the kidneys physically is slim, unless he was just cranking down and really beating the shit out of him.
The more likely scenario is that the dog received a UTI through oral, manual, or anal contact with Kero that went untreated and had progressed into a kidney infection.
Having anal sex with the dog may also be a contributing factor if there was damage to the intestines or prostate. An enlarged prostate (possibly damaged by the sexual abuse) can block the flow of urine, which will lead to kidney issues and eventually failure.
Basically, any form of obstruction or introduction of bacteria to a urinary tract can lead to kidney issues, and kidney issues when left untreated in dogs tend to kill quickly and painfully.