Hello @lisa23!
I agree with Limeberry- Malachite is a great way to go! You could also consider Crystals for Protection (Black Tourmaline, Obsidian, Tiger’s Eye) to help keep her safe during the surgery 
She is likely exhausted from the stress- very understandable! So another approach could be crystals that help soothe anxiety- freeing her natural energy in the process. I suggest Aquamarine, Amethyst, and Lepidolite 
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As for baths- I would check with the doctor about care procedures post-surgery. Sometimes they forbid getting the area wet for a certain amount of time after the surgery. Same with oils- might not be a good idea to apply any directly to the area.
However, aromatherapy offers many benefits and breathing in the gentle scent of herbs such as Lavender oil can be very calming. Tea tree oil is believed to have healing properties 
I would also suggest herbal teas if the doctor allows it- Chamomile, Lavender, and Lemon Balm are very soothing and can help ease anxiety. You might also consider teas that help with the healing process (Elderberry, Echinacea, Garlic)! 
Just be sure to check with the doctor to to make sure any ingested herbal supplements won’t have a negative effect on the medicine.
Wishing you niece a successful kidney transplant and speedy recovery- blessed be! 