"well, death has a purpose sometimes. It helps you see the good things..."

I don't find this repugnant. Your friend clearly would not kill their grandmother in order to learn a lesson. I think they are simply looking for the silver lining, and looking away from the horror. This is a fair strategy in this case, because the fact is that nobody was able to prevent your grandmother's death. Being rational means being able to look at the world as it is, but it doesn't mean you're never allowed to stop staring at the worst parts.