If long hours bother her a lot, she may try looking at completely different field because BOTH cardiology and card surg involve very long hours.
Even if they limit the resident hours to 80 hour a week, most attending physicians in those fields work well over 80 hours a week and the fellowships for both are pretty tough. I have seen cardiac surgeries that last over 10 hours - your girlfriend has to be prepared to stand for that long, sometimes without a break, and concentrate on her job - if not, the patient could die. Cardiologists probably miss a whole lot of family events as well - they have to care for the same sick patients too - you can't just walk out of work at 5pm if your patient just went into some bad arrhythmia.
I wonder why is she so attracted to these fields? She should really talk to people in her school in these feilds and find out exactly what their job entails and how it affects their life at home. Surgery and medicine are really very different too, so she needs to figure out which one is more right for her.