That's scary stuff. Not that long ago a guy at work suffered heart failure. It wasn't a heart attack, his heart just stopped. Fortunately we have AEDs in multiple locations and plenty of people trained in first aid. They were able to use the AED to save his life. He now has a difibulator imppanted in his chest in case it happens again.

Is she doesn't have any warning, there may not be much you can do.