"I can't wait to get started; I've managed to get the things you've asked for. I've got salt to make a saline solution; I got that from my local chemist. He didn't ask a single question, not one. [...]"
Surely, the aim was to obtain all the items as inconspicuously as possible. Why buy the salt from a chemist? Would that not seem rather odd? All that was needed for their purposes was regular table salt. Surely, she had that at home already ... or could have gotten it with a regular grocery shop...
Also, the events that follow do not make for the perfect crime.
Again, this would have been a perfectly good story if it did not try to force itself to be based on Agatha Christie.