Would drinking a little extra water really help??

I'm a little concerned about what my best friend's doctor told her to do. Certainly, I am not a doctor, but what he suggested to her doesn't make any sense. According to her blood test, her Gfr was just a few points below the parameters. He suggested she drink a little more water. My friend is nearly 66yrs of age, & very healthy. She eats a high salt diet. I know for a fact that drinking too much water can reduce the natural salts in the blood which can be detrimental., & not drinking enough water can make those salts higher. 

Not too long ago, she was having trouble with her feet. The same Dr. told her to buy & wear Birkenstocks. As it turned out, she had calcium deposits in her feet, & once they were eliminated, her pain was gone.

My main question here is regarding the water.