What is her attitude about what happened? Does she feel that she neglected her child?
I don't think it's as simple as either report or keep your nose out.
If she neglected her infant then action had to be taken. A child relies totally on its caregiver and has to come first. Because of their total vulnerability.
If she changed because of that, if she understands what went wrong, if she feels that she would never go down that path again, etc etc, then I would say don't report. Wait. Watch for any sign and then act.
If, otoh, she doesn't care that she neglected the other child, doesn't think she made any errors, doesn't intend to do anything differently, etc, then yes, report. Because she will need someone to be there before the birth.
It isn't always good enough to be reactive in a situation.
So really - yabu/yanbu? It depends so much on the specifics.