Hi, Danielle.
My mom had a kidney removed when she was 19 (in 1950, due to severe infection), and she is now 78! She has never had problems from living with one kidney, and she doesn't have any diet restrictions.
I also have only one working kidney. I had kidney failure due to diabetes, and then I had a kidney transplant. So I've been living with one kidney for over 10 years. I do have some issues, but it's because I have a transplanted kidney - not because I have only one kidney. My kidney has been very healthy for most of the 10 years, with a perfect creatinine of 1.0. Now I have ovarian cancer, and my creatinine runs 1.3 now because of apparent damage from chemo. That's still pretty good, though!
I think the main thing for living with one kidney is hydration. Your sister should drink lots of water and keep that kidney hydrated!
Best wishes and God bless,
Joan