Dear Jesus, Divine Physician and Healer of the Sick, we turn to You in this time of illness. O dearest Comforter of the Troubled, alleviate our worry and sorrow with Your gentle love, and grant us the grace and strength to accept this burden. Dear God, we place our worries in Your hands. We ask that You restore Your servant to health again. Above all, grant us the grace to acknowledge Your holy will and know that whatsoever You do, You do for the love of us.
I find that a simple sincere heartfelt prayer works best.
If you pray directly to God and thank him for all the good things in your life/your friends life and then ask for strength and comfort for her and her family to be able to deal with her illness and to help you be there for her and to be inspired to know how best you can help and support her. Ask for anything else you would hope for and close the prayer in the name of Jesus Christ and end with amen.