Another possibility is that a friend may be giving her the wrong advice. You never leave babies, or small childred alone in the house. Not even for a few minutes. When kids are older and they cry at bedtime because they want the parents to run back in the room, that’s the time to let them “cry it out”, not when it’s a newborn. Do you know someone who’s had children who can sit down and talk to her? Can you talk to your BIL? Post partum depression is always a possibility, but I don’t know enough to say.