Hiya Secretflames! (^.^)
Try Buddhist mantra "Om Ma Ni Pad Me Hum"
It most powerful mantra.
Chant it 5 sets of 108,before surgery and after surgery.
If he die his heavy sins and bad karma will be destroyed and he go to peaceful after life or peaceful rebirth.
If he live he will have good health.
But have faith in words as you say it. :)
I also recommend Medicine Buddha mantra,,,,but I no know it.
and mantra of of Guan Yin. Chant her mantra 5 sets or 108 "Na Mo Dei Bei Guan Shi Yin Pu Sa" If he die Guan Yin will guide him to a peaceful after life or peaceful rebirth.
If he die and go to a hell chant the mantra sutra Dei Bei Zhou or Chant mantra of Kisgarbha Bodhiasttva "Om Na Mo Di Zhang Wang Pu Sa" he will guide him to a peaceful rebirth.
maybe it help.