I think this needs to be added. While she DOES have end-stage kidney disease, which requires regular dialysis, the issue with transplants is that you need to take immunosuppressants to fight off tissue rejection. Barring cloning, it's still a part of someone elses' body that's in them, and their body sees it as an invader, no matter the function of the organ.
Transplant recipients have a higher rate of negative outcome of COVID due to the immunosuppressants hindering the body's ability to mount a defense.
Not to mention have a higher chance of contracting COVID after exposure to the virus because of the weakened immune system.
That's why they're insisting before the procedure. So that the immune system has a chance to learn before they weaken it.