Talk to Nephrologist on Kidney Problem

Assalaamu Alaikum respected doctors. My sister, one night drank coca cola and have not drank water and went to sleep. When she woke up the next morning to pee, she noticed her pee was red and felt burning sensation. She was not on her menstrual period. She drank lots of water after that then peed with normal color just having some kind discomfort. She went for check up, did urine analysis, rbc and protein were negative, only wbc was positive. And she did also urine CS, result was no growth. But she did ultrasound KUB, and the result was the left kidney has mild echogenecity grade 1 nephropathy. What does that mean? Is it serious? Reddish urine was only once. And was it caused by drinking coke?