I am sorry to come on and find your hen did not make it. It is a shame to lose any let alone to mites.
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Some garden plants are sold without any kind of warning that every single part of the plant is highly toxic as well. I learned that the hard way myself. Lilly ate a bit of a tiny ground cover plant and died from the toxicity of it.
I am so very sorry you are dealing with losing her in this way. As enola said your heart was in the right place when you were trying to help her.