This is a toughie. First, try to work it out w/the potential nanny first before you post something publicly on a public forum. Something could have happened so try to contact her and see what she says. If she doesn't respond within a week, then you CAN post something, but it really is your call.
As a parent, I would only post something negative about someone if they were a horrible childcare provider. I.e., negligent, abusive, etc.
Not for someone who was a no show for an interview.
If she was a no show for a job and didn't call, then yes, that would be a whole different thing.