I think the OP has been pasted enough.
Don't know about everyone else, but I read A4 sheets pretty much in one go (it's my job) so I could have seen all those details just in the process of turning the letter over to hide it (which, incidentally, would somehow have felt 'worse' than leaving it untouched).
I know, for instance, from passing through the staffroom briefly, that our school has a child with an onion allergy, that two others have an epipen, and that there was an ed psych in that day to assess another. Didn't mean to read any of it, but it wasn't hidden.
I wouldn't necessarily remember every detail from a 2-second scan, but I would know that it had all those details, IYSWIM. And you can't un-read something once seen.
If I was shown to a desk with a sheet saying SAFEGUARDING on it in big letters, and was there to fill in my CRB details, I might even have assumed I was meant to read it - like a fire notice, or a note about how to use the computer.
Yes, she should have asked for it to be moved, but was possibly too embarrassed, having realised it was something so private.
No, she shouldn't have put so much detail on here. Live and learn.