Landscapers and interior decorators who plop plants everywhere are among the worst enemies of dogs. If a plant is pretty, they use it, and never mind whether it's poisonous or not.
A long, long time ago, I shared an apartment with a guy who was a plant nut. I thought his plants were cool. When my little mutt started chewing on a dried up bit of plant (which I later found out was dumb cane), I thought nothing of it as I took the plant piece away from her. Fifteen minutes later, her whole face was insanely swollen up and the swelling was moving down into her neck. Didn't take a genius to figure out that her throat was going to swell shut and she was going to suffocate. As long as I live, I will never forget that frantic race to the vet. When I got close to the vet and hit stalled traffic, I jerked the car into a no parking zone, jumped out, and ran with my dog in my arms to the vet. They shot her up with steroids and Benadryl, which stopped the swelling and saved her life. As soon as I got home, I dumped every plant in the apartment in the dumpster and told my roommate that if he ever brought another eff-word plant into the place I would move out the same day. He started to get indignant but shut up when he saw I meant business. I have never had plants in my home since then, and I've routinely removed landscaped plants.
When in doubt, rip it out.
And as I've said a million times, just because something is natural doesn't mean it won't hurt or kill you or your dogs.
Here's a couple of websites with lists of poisonous landscaping plants. The first one focuses on desert plants.
Did the ER vets do bloodwork on Seiran? If they did, what were her sodium and potassium levels? This is important since she was never tested for Addison's disease. I have no doubt that she was poisoned by the plant, but unfortunately a dog can simultaneously have more than one medical problem that sends her to the ER.
Hope she makes a full recovery! Please keep us updated.