So sorry for your loss. I know it must be hard, but please don't blame yourself. His health issues would have been there regardless of what you did. Even if you did EVERYTHING 100% spot on perfect, which NOBODY does, he still would have had health issues. Falling is an accident and can happen to anybody, even if all safety measures are taken. Don't beat yourself up, be there for your fiance and mourn the way you need to.
My thoughts are with you and your fiance.