Bringing this up with a Witness is just using a tragedy to score a point.

It can be, and that is the wrong use of it. Some Witnesses have buried a parent, spouse, or child who refused treatment that is now permitted, and the first response to that is grief, not argument. Raise it only when the conversation is already about how the organization reaches its teachings, and ask rather than assert. The point is not that they were foolish. It is that they trusted a teaching that has since been withdrawn.