Now, (as i learned in prior psych classes) psych is supposed to be a real hard science, research and use your critical thinking skills to evaluate and form an opinion.
I don't mean to make light of your dilemma, but I had to laugh when I read this. I was a psych nurse for years and it is about as inexact a "science" as you will ever run across. There are those in academia who do not consider psych a legitimate discipline because ethical concerns (messing with people's minds) and subjective evaluation (as opposed to objective data--lab tests, CT, MRI, etc.) make research such muddle.
At any rate, if you have access to websites that cite reliable information regarding this misinformation, by all means, share it in the way a previous poster suggested. "I thought this was a danger too, until I found these contradictory reports." Be sure that you tell her you will do whatever she wants, so she knows you're not trying to undermine her authority. And then just go with whatever she decides.
If you approach this like you're just sharing something you thought she would find interesting, the idea might seem less threatening. She's only trying to protect her kid and the more you project the idea that you're on her side, the better chance you have of letting her know she doesn't need to worry about this particular concern.
One more thing. Think of her situation. She's the parent of a special needs child. There are so many things over which she has no control whatsoever. Sometimes it can be tempting to latch on to a concern where you feel a little bit empowered. "I may not be able to change my daughter's mental capacity, but, doggone it, I can keep her from getting poisoned by her microwave meat loaf."
You sound like a caring and concerned person. Let us know what happens.