I'm open to think she operated alone, personally.
In the cases of previous baby-snatching ('womb raiders' or women who steal newborns from their parents), has there been a case where the man has been fully informed prior to the event, and been involved, or even caused, the killing himself?
To my memory, the closest we've seen is men who knew after the fact and were negligent with the information, not informing LE; those who have been ignorant and seemed to fully believe their partner had been pregnant and given birth, no matter how many clues there were against that; and even those who have been suspicious, but not enough to call LE about the situation.
But I don't remember right now a man and woman both killing a mum for the baby, or a man knowing about the plan and really helping their partner during the actual killing, even if only to restrain/threaten the parent.