Talk to Nephrologist on Kidney Issue

My had a heart few years back, and due to elevated creatinine test results we consulted nephrologist & through ultrasound & radio isotope test he came to this conclusion that my father's right kidney isn't working completely for unknown period, while he had no symptoms which suggest it, but as we are recommended to monitor creatinine level as precaution every few months, we recently tested his creatinine level which came at the same level which he has for quite some time which is 1.34, but the lab on its own also gave us the result of eGFR which is at 47.7 levels, so want to know is this normal for person having one working kidney or is it should be concerning?!