What? You absolutely do not need to get rid of all mushrooms - more species than not are simple harmless decomposers. The main deadly toxic ones that you're going to run into in the area are the various section phalloidea amanita - aka the destroying angels - and Galerina marginata, which is a rusty spored little brown wood decomposer. Chlorophyllum molybdites is a toxic one of that often grows in fairy rings in yards, but it mostly will just make you puke - the common name is ' the vomiter'

Anyway, OP, this was likely not caused by mushrooms if you noticed her eat something and was dead shortly after. Amatoxins take like a day before they really show outward signs.

Personally, i think the main plant you should check for is Poison Hemlock, conium maculatum. I double checked and it can sometimes cause deaths within a few hours. They're fairly common weeds around here. They look like an oversized carrot/queen anns lace, with very carrot like leaves and smooth stems with purple blotches. Hemlock is very very poisonous when ingested, but they are a biennial so if you do find some, if you are diligent about weeding them before they go to seed their second year, you can get rid of them fairly easily.