Just because a receipt can be shown for mushrooms doesn't mean they are the mushrooms consumed on the day.As long as she paid by card, her purchases can be verified no problem. And I think police will be able to access her bank records to check that out - as she is a POI/suspect and cooked the meal. They'll need to do that as part of the verification process.Moo
If one is cunning enough to kill people this way they are cunning enough to think through the details and alibi's.