If it hasn't metastasized, it won't be a big deal. I had renal cancer in early 2020.
I heard my whole life, if you ever donate a kidney it's relatively simple surgery. Not that bad really. The afternoon after mine was removed, I remember thinking I'm not so sure I agree right now.
But that's the key... if it's contained within the one kidney, and the other kidney is healthy, there isn't really a good reason NOT to just remove the kidney. They could try to just remove the tumor, but the chance then of a recurrence is higher.
And honestly, if there's a mass on her kidney they'll probably remove the kidney anyway, even if it's not malignant. But other than her doctors will keep a close eye on kidney function for a while, if not the rest of her life, you really don't notice any difference between one kidney and two. So pray that it's contained in just the kidney.
Aries
~Sign of the Rams