My dad had a stent put in to help pass a kidney stone. a few days later,he had severe pain and developed a fever. blood work showed he has sepsis. ?

Sepsis: He really should be on antibiotics given IV. So this would at lest require that he spends a few days in hospital. Sepsis can progress rapidly and affect other organ systems so it has to be treated.

Pyelonephritis.: Seems like the stone caused a kidney infection before it was removed. The blockage of urine flow out of that kidney would have allowed bacteria within the urine to set up shop and cause an infection. But the removal of that stone should help eradicate that infection.