Talk to Nephrologist on Kidney Issue

My father has only one working kidney, & we found out that few years back when he had a heart attack & Doctor ordered UCE test, and upon elevated creatinine he was advised to consult a nephrologist which advised ultrasound & radio isotope test of kidneys, where we found out that due to stone in his right kidney it has stopped working completely, he didn't feel any symptoms nor does he has any symptoms related to it, but as we were advised by doctors previously that regular monitoring of creatinine is necessary hence we just had him tested upon which the lab voluntarily done eGFR test with creatinine, & his eGFR cane at 47.7 so i want to know is it normal for a person who has a one working kidney?!