The only meal all the victims ate was the one provided by Erin Patterson.
Initially in the hospital they assumed it must be some sort of food poisoning cause by the meat.
After liver functions and other tests showed something else was happening, the thought of mushroom poisoning was raised, so it was important to know if the mushrooms had been foraged.
There are quite a few poisonous mushrooms. DCs are quite distinctive in appearance, and also the most poisonous of mushrooms, and it was not until a toxicologist raised the possibility of DC poisoning that they realised this was probably what they were dealing with - and that it was, frankly, impossible for the victims to have eaten DCs if the mushrooms were bought from shops. But also, if they had been somehow put into the supply, it was a major public health hazard.
How is it possible that they all got poisoned but she didn't? Or are the defence maintaining that she was also poisoned? How did she prepare them that made it possible for only one to not contain poison?
She made individual Beef Wellingtons, like little fancy pasties. The mushroom duxelle or paste that lines each BW was full of Death Caps, designed to kill them - not just make them sick. Her mushroom paste was obviously made separately. She put theirs on four grey plates, and hers on an orange one, to be sure she didn't eat the wrong one.
There really is no way she could have just lucked out from being "a bit poisoned".
I don't think she'll testify, but who knows.