There is no healing prayer in Buddhism. Otherwise all will be healed by just chanting the "healing prayer". A lot of Asians claim to be "Buddhists" but just in name only, for they do not even know who the Buddha was. I take it that your friend is a "practising" Buddhist. Then you got to find out which tradition she practises. Different traditions have different "prayers" for the sick.
"May the Buddha, Dhamma and Sangha give you strength to face all adversity, and may you regain your health in due course."
Hope this helps.