I had open heart surgery when i was 4 so im sorta familiar with this situation myself, though that was at such a young age i didn't know really what was going on at the time. Now im 23, even being healthy its kinda a scare considering some of the issues that could come up out of the blue with the long term outlook of the fontan procedure and the very real fact that nothing is know about how a heart will take the abuse past the 40 year mark and beyond. Dont get me wrong,its not as if i spend all my time thinking about it as that be no way to live, but for me its also kinda the not knowing if you get what i mean. I guess its hard to say what i would do, depends on how grim the outlook was.
Sorry to get all deep, hope yout cousin's gram pulls through, and remember its better that shes having these thoughts because they mean shes fighting and not giving in. All the best.