Well, its interesting that the rectified time I have for Dad shows his natal asc to be 15 Capricorn. If that is the case, then the event chart has been made to order.
The 8th house of surgery is indeed ruled by the Sun and trine Saturn shows the surgery event to my father's significator Saturn. I am not sure about his ruler being retrograde and cadent though. That is a very weak position for him however the mutual reception between him and Saturns lord, Mercury is supportive with Mercury strong at the ascendant.
The surgeon who is the Moon is angular, I will have to ask if the surgeon is female or just very popular/prominent.
Anyway, the moon last squared Jupiter so there could be some unexpected blood loss or blood pressure (same thing) and then it will be up to Dad's heart and whether his body accepts the valve etc.
My father is quite religious so perhaps he had some prayers chanted out by his B'ahai friends at that time. Cool.