Christian Scientists reject many things that might be viewed as common medical intervention. It is my impression that to talk them out of the consequences of this position, would be to talk them out of Christian Science.
I'm not saying I agree with their position, and I'm certainly not an adherent of their religion, but I can see some sense in the rejection of many common medical interventions too. In fact, some of the decisions I have made may lead to my "early" death in the eyes of some. Those are decisions I've made, some people won't understand them, and I can understand not wanting to argue the point---though I'm sure I may have to if it comes to that.
Sometimes it's difficult to know what to do to help people, but if you want people like that to confide in you, I'd guess it's important for them to think you'll understand their point of view. If, in this case, that means becoming a Christian Scientist, then I guess there's not much to be done. I think I would write something like this to the wife. At least you could offer to help etc.---of course, you know that. Tough situation. Sorry for your loss.