This - there's an intervening act which breaks the chain of causation - the woman ate the sweets knowing what they were and the risks. So in addition to proving the act, and the fact that eating the sweets caused the death (90 year old people die, how can you demonstrate what she died of?) you'd have to prove lack of capacity in the woman to make the decision to eat sweets.Did the old lady have dementia or otherwise compromised judgement/memory? If not, well, that's on her. She was an adult that made her own decision. Hell, a coma isn't the worst way to go, so maybe that outcome was intentional.
If she was impaired, then yeah, it's on the nurse. OP and others covered that angle already. My point is that even people in retirement homes can (don't always, but can) still retain their own agency and that deserves consideration and respect.
Even if you could prove all that, does smuggling in sweets fulfill "so blameworthy as to warrant the same criminal liability as that which the law imposes upon a person who intentionally causes a crime" and was "imminently dangerous and presented a very high risk of death"? There's a difference between "you shouldn't have sweets, you're diabetic" and "giving you this is tantamount to giving you cyanide". High blood sugar isn't automatically or even usually fatal. I'm far from convinced that the act would be criminal even if you were able to demonstrate what took place.